I'll hook you up, brutha:
Car Audio for Dummy's (jesus I hope I don't get sued)
First, is it your intention to only have the subs powered by the amp? (what's going to power the other speakers?)
If so, a 2 channel amp is sufficient. If you're intention is to run the standard 4 speakers and then 2 subs, you'd need at least a 4 channel amp (just going by your "single amp" statement.
Second, I agree that Sony XPlod isn't the best for your application. From my experience, the XPlod (subs at least) are loose and boomy, perfect for the rap connoisseur. I have 2 Polk Audio 12s that I absolutely love. The 10s are very tight and are more likely to fit your application. As for amps, it really depends on how much you want to spend. Things have changed so dramatically over the years in car audio that I don't even know who's on top anymore. Used to be that Rockford Fosgate and Orion shared the marketplace in "Bad *** amps". They were clean (Low total harmonic distortion;better sound), 2 or 1 ohm stable (read: more speakers) and powerful. One thing to consider regardless of what name amp you get is total wattage. Nevermind what's on the box or on the case of the amp, that's crap. Look for the RMS rating. The RMS (root means squared) is the wattage that the amp will consistently put out. The wattage they put on the box is "peak" wattage, which is crap because anytime it's over it's normal level, the distortion goes up right with it.
Bridging is combining two or more channels to act as one. If you have a 2 channel, 50 watt amp (RMS of course), theoretically each channel would put out 25 watts and you could run a speaker on each channel. If you only had one speaker to run you could "bridge" the amp and thereby get a one channel amp that puts out 50 watts. Are you picking up what I'm putting down? It gets far more difficult as we get into ohm loads and wiring in series or parallel, but I think we've had enough for today. Besides, I'm going to be known as the "long post guy" before too long.