I am to understand that it doesn't matter at all. Yellowsn0w is a package that is delivered at boot. That is to say that when your phone is turned off, it doesn't know which sim is in it. When turn your phone on, yellowsn0w is delivered at boot and is running, ... now it doesn't matter which sim is in for the phone to work.
After you install yellowsn0w, be sure to power off.
don't stand by or quick respring, or any such baloney, ... POWER OFF
Hold down the power button until it says slide to power off then slide.