you don't combine them, the F & R signals (per side L & R) get combined in the amp for the door/subs. Therefore, in order to get correct function for each sub/door speaker, you need the F & R low level inputs from the h/u for each side (L & R) at the amp.
For example, w/ or w/o rear speakers in a Boxster, if you fade 100% to the front speakers, there is no sound from the doors. Same if you fade all the way to the rear - no sound from door/sub (doesn't matter if you have the rear speaker option or not). You will only get full sound (not freq as the amp has a crossover) from the doors when the fade function is set in the middle and thus why you need both the F & R of each side from the h/u at the amp.
Clearer?
