I want to flash my TSOP w/ a split 4977/retail bios for use on XBox Live later on. For now, without a toggle switch, which half of the TSOP boots up first? I'm guessing the top half, and that's where I want to put my multi bios until I get a switch, but don't wanna risk anything until I know for sure.
Since I won't be able to toggle until I get a switch, how can I be sure that the 4977 half will boot?

I don't think(i could be wrong) you can use a 4977 bios with a retail bios. i'll tell you why little bit later. For example, Xbox TSOP is 1mb, its devided in four banks which has 256kb bios in each banks. which could be the same bios or multiple bioses. I read somewhere that xbox origianally boot the last bank first, and then it verifies with other banks as its booting. And it only VERIFIES and NOT uses other three 256kb bios images. Thing it verifies is somthing like the version number for its motherboard. So to come to your 4977 bios is a multi bios and it has completely different code. And if i'm correct it will fail to boot the xbox. only way to combine is to use the old "evox 2.5" for example, and not evox D6 bios or any multi bios.