1. Yes. This will require extra i/cs
*Do I need a external x-over?
*Do I need new ICs?
*Do I need new Speaker Cable?
*Is this whole thing just dumb?
2. If your pre has dual outputs you will need a second set. If your pre does not, you will need Y cables.
3. You will need 2 sets of single-wire speaker cables.
4. Dunno. Add up the cost of 1-3 plus the value of the 2 amps. Maybe could be better put to use buying a single better amp OR sell one of the amps, get a second to match the one you kept. Then you could eliminate the cost of a cross-over.