Combining Two Amps?

Yes! Dual amps are very cool. There are several ways to go about this;
1) Use a pedal with 2 outputs, one each to each amp.
2) Line out from one amp into other amp
3) From a mixer to amps( guitar input to mixer, left and right channels out of   mixer into amps.

I use an unpowered Mackie mixer to send my guitar to a Marshall 50watt soloist, From the Marshall I run a line out into a Behringer 45watt acoustisonic amp. The Marshall is behind me stage left, the Behringer is bhind me stage right. I angle the amps slightly inward and make sure that they're at least 6 feet away from your vocal mic.(feedback!!***)

The only effects on the amps I use are digital EQ and reverb.

The sound is equal(hopefully) from each amp.

The pedals I use on the stage floor, pre Mackie mixer, are double EQ( one on continous, one used in geese formation for solo's) compressor/sustain and phase, all Bose pedals. I especially like the phase pedal because it combines the spacy flange sound, a bit of tremelo, and chorus too. Keep in mind this is my acoustic set-up, my electric is a whole new monster.

   As far as cables I use all monster cables( great quality and lifetime replacement policy). They're pricier than the usual cable but if you play out a ton, they're well worth the initial expense.

   Hope I've suffiently answered your questions.

I read in one of the guitar mags that either white zombie or smashing pumpkins used to split signal to a regular amp and a bass amp. I've never tried it but always thought that might be cool to try. Maybe use eq after the signal is split to beef up the sound. Also I read somewhere about using a stereo chorus and spliting the signal so that the two amps go in and out of phase.