Hiya,
You certainly can, although on multi-pre-out head units you'll be able to fade front/back and control the sub level. Certainly the case on my Sony M770R
Just split the single pre-out using suitable phono Y connectors to the necessary amps. then you'll have to adjust the levels on the amps themselves to get the balance right.
I put 6x9 in the front doors of the tdi, I just made some MDF frames up and fibreglassed them to a smooth appearance, attached these to the doors, not the card itself and then modified door pockets to fit around later on. Took a good couple of weekends to finish, but the sound was awesome.