skinnyazz, there's a simple mod you can do to dual boot a tsop:
1. Create and flash a 1MB bios where the first 512k is the retail, and the last 512k is the other one you want to use.
2. Use a spst to short the msb (a19) of the address to gnd when you want to boot retail. This signal can be found on pin 1 of the tsop, and is also noted on bunnies rom pinouts pic. The point in his pic is a via so you can grab it on the top side of the board next to the MCPX if you don't want to solder to the tsop itself or pull the PCB. You can solder the other side of the switch to the sheild or any other gnd you have handy.
3. Don't forget about the HD lock and dashboard names, I would suggest using an alternate dash name bios or one of the F boot versions floating around. Keep in mind that the current evox uses the md5 of the first 256k to identify the bios so don't get confused when you have more than one bios loaded.
This works because the xbox boots from the last 256k. So by shorting the msb, you limit the size of the rom and force it to boot from the second 256k. However, it appears to be a full 1MB to the xbox since it doesn't know it can't toggle the bit. You can extend this idea further by adding a switch to a18 as well for a triple boot.
simple low hassle dual boot for all folks that realize the only "modchip" you need comes in the box...
enjoy