Does anyone know what the pros/cons of putting a coupler/splitter on a sub pre-amp output would be? Would putting a splitter there weaken the signal any? I want to run 2 subs off of 2 different amps. I'm thinking it shouldn't be a problem because the signal doesn't get altered (amplified) until it goes through the amp anyway.

Not sure what you're getting at. Just avoid hooking two (or more) pre-amp *output* signals directly together, otherwise you can do anything else you want.