I wrote this tutorial a couple weeks ago and submitted it to xbox-scene, but it has yet to be posted. This goes into a lot more detail, but has worked flawlessly....
Use a "Multi" Type Bios in a Dual Booting TSOP
* This mod will not work on a v1.2 xbox because the tsop is only 256K. DO NOT attempt on a v1.2 xbox.
** If you do not already have a flashed tsop I would not recommend doing this modification to your xbox. If you dont know what a flash or a tsop is, just stop, right now.
A Little Background
Alright kiddies, here we go again, another quality (or not) Dummy tutorial from The Dude. But now this is a little more advanced, but not so advanced that even your dumb ass can't figure it out.
So, you would like to flash the xbox tsop, but you want to use a "Multi" Bios along with a retail bios to be able to dual boot. What's a multi bios you may ask? It is a bios that you can use on v1.0, v1.1 and now v1.2 Xboxs. The Executor 4974 and newer are all a multi bios as well as the EvolutionX D.6. Very Convenient for some, but for those of us that would like to use it with a retail bios on our TSOP, it just won't work. Why? We won't go into any huge detail here, but to make a long story short the retail bios and the multi bios are too different to work together. As you should all know by now your xbox tsop is 1024K. Each tsop contains four 256K BIOSs, all alike, or different. When your xbox boots up by default it uses the last 256K of the tsop as the bios, but soon after the xbox "double checks" (lets say) the first 256K of the tsop. If the last and first 256K of the tsop differs too much the xbox will invalidate the bios and the boot will fail. With BIOSs like Executor 4973 and older as well as Evox 2.6, 3.6 and older the xbox had no trouble booting with these BIOSs along with retail because they were "Version Specific" and were very similar to retail. So with all that out of the way, its time to get to work.....
The original method for switching a multi bios tsop was to solder a wire from the A19 test point on the motherboard to a SPST switch, then a wire from that switch to any ground point on the xbox motherboard. By forcing A19 to ground you cause a 0v condition causing the xbox to only be able to see the first two 256K parts of the tsop. A custom bios needs to be created for this purpose. The bios looked like...
Original+Original+Modified+Modified
And of course the Modified bios was a version specific type. When the switch was closed (on) A19 is grounded and the xbox can no longer see the last two 256K parts of the tsop, hence booting the original bios because it uses the last 256K of the tsop that it can see. When the switch is open (off) the xbox can see the entire 1024K of the bios (as normal) booting the modified bios. There would not be a conflict between the last 256K and the first 256K, if you are using a version specific modified bios.
Creating a Dual boot Tsop using an original bios and a multi bios
Tools and Supplies
Soldering iron
Solder
Flux (if necessary)
Desolder Braid (if you're a dip shit)
SPDT Micro Switch (or a big one if you like that)
Wire (thin as possible)
All the damn tools you use to take your xbox apart, if you don't have them by now, or don't know how by now, just stop now.
So now we must have a way to split the xbox tsop in two so when we boot one way or another the xbox can only see one half of the tsop at a time. Before grounding a19 would cause only the first half of the tsop to be visible, but ungrounded the xbox can see the whole tsop. In order for the xbox to only see the Second half of the tsop we must force 3.3v to a19. It is just that easy.
I highly suggest flashing your xbox with a 1024K modified multi bios of your choice first. MAKE SURE YOU REMEMBER WHAT VERSION YOUR ORIGINAL BIOS IS. Whatever method you decide on how to do this is entirely up to you. 007/linux tsop flash is the fastest and easiest; a 29 wire homebrew mod to boot then flash is also a good method. If you don't know how, there are plenty of tutorials out there that show you how to do this. Now that you have your modified bios on the tsop, you should install EvolutionX or another dashboard with the means of flashing the bios easily.
Now we will create a 512K bios because you will eventually only be flashing half of your tsop. Since you already have a modified bios on your xbox you will want to make a 512K original bios. It is VERY important that you use the same type of original bios your xbox originally had. If you have a 256K version of the bios you must copy it once to make a 512K bios.
The simple DOS command for this would be....
copy /b original.bin+orignal.bin original_512K.bin
If you have a 1024K version of the bios you must split it in half. There is a good tutorial on how to do this on xbox-scene using win hex workshop.
Installing the switch
So now you have your SPDT switch and it looks something like this....
Cut yourself 3 pieces of wire probably about 10 inches long (25 centimeters for you European folks) and solder them to the 3 points on the bottom of the switch. You want to solder the center wire to the A19 test point on the motherboard. Now solder either one of the wires on each side to a ground point on the motherboard. The other unsoldered wire you will now solder to Vcc, preferably the big fat point on the LPC. Remember which point on the switch you soldered to either Vcc or Ground. Here is a really stupid looking diagram, but you get the picture. Also two pics of the motherboard showing you were A19, Ground and Vcc should be......
So now you have your switch wired in. If you are confident with your work put your xbox back together, if you're not then leave it so you can easily disassemble after you test it. Mounting the switch is entirely up to you. Once again there are many tutorials floating around on that subject, just be creative. When you power up, boot with the switch in the grounded position to make sure that the first half of the tsop boots your modified bios. Then power off and boot up again with the switch to Vcc and make sure that works. After you have confirmed both ground and Vcc are working, you can now power down and set the switch once again to ground. Hopefully you have EvolutionX or another dashboard you can flash your bios with installed. Do what you have to do to put the bios on the xbox so you can use it to flash the tsop with. Now you have your 512K original bios ready to flash to your xbox and you are booted with A19 grounded. Flash the tsop. Now power off, if it hasn't already done so, and power on with the switch to ground. If it boots the original bios and you see the MS dashboard, you were successful with the flash. Now once again flip the switch so A19 is forced to Vcc to make sure your modified bios is still working. Now you have original MS bios while A19 grounded, and you have your modified bios when A19 is forced to Vcc. You can also force A19 to Vcc and flash the second half of the tsop with a new 512K bios in the same fashion.
A few notes to remember, original BIOSs will not work with an unlocked hard drive. Also Make sure that you have your MS dashboard as xboxdash.xbe and your alternate dash (Evox, nextgen, whatever) named differently (evoxdash.xbe, blah, blah). Make sure also if you have a replacement Hard Disk, that it is in a locked condition, otherwise the retail bios will give you an error. Now you can enjoy your xbox and all of its potential using your modified bios, or you can switch and run the xbox like it is a normal retail xbox. In retail mode you can use xbox live, just make sure you dont use xbox live while using your modified bios.
Alright, now its time for the Thank You
First off thanks to all the xbox homebrew developers around the world, without you none of this would be possible.
Thanks to Jujupinto85 for his original multibios guide, and thanks to Ant2483 for his forum post.
And last but not certainly least, 101^ for putting up with all my paranoid shit for the last month, and all his incredible support.
Sorry for the lack of pics dudes, wasn't quite sure how to put them in, pretty new to this forum deal here.