Hi all, greetings from Brazil, first message here, don't forget to take a look on
http://www.tabajara-labs.com.br 
)
Well, the method is great, but indeed there is a simpler way:
1 - Buy a SST39sf040a instead of a SST39sf020a - it has double the capacity
2 - take the two images that you would flash (one for the original and another for the modded) and join it toghether with a file joiner or the old DOS command copy file.bin >>file2.bin /b
3 - Flash the SST39sf040a
4 - solder it in place but lift the higher adress pin and tie it to ground thru a 10Kohm resistor
5 - use a SPST key with one pole tied to +vcc and another tied to this pin
Here you have a simpler mod

)
Explaining:
The chip is binary adressed. We have, as an example, 2 bits of adressing (yes, I know there are more bits, STFU). Call them A0 and A1
A1 A0
0 0
0 1
1 0
1 1
If we add another bit of adressing, we have doubled the possibilities
A2 A1 A0
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0|
1 0 1| Look here! The same!
1 1 0|
1 1 1|
So if we double the memory, we just add one adress line to the chip. So we can use this adress line to create two "windows" in the chip. One will be the original firmware, another the hacked one.
Of course, if you add two adress lines, you can quadruple the memory. So if you want to use 3 (or 4) different firmwares, you can use a higher memory chip (I have to check, but I believe there is a SST39sf080a which has 4x the memory of the SST39sf020a) and do the same procedure with the two higher adress lines.
I'm late to work. If there is an interest, I can write a PDF later.
See ya
Alexandre
(still with the old XBOX and loving it)