You'll be a little bit underpowered connecting two 400W speakers to a 900W amp, but as long as you don't crank it all the way up you should be fine.

Check the impedance, though. You do *not* want the impedance to go lower than what the amp is rated for. Here's a website that will help you calculate what kind of load you're presenting to the amp:

http://www.bcae1.com/spkrmlti.htm

That'll tell you more than you ever wanted to know about speaker wiring.

Oh, and you should note that the two speakers will reproduce frequencies with different efficiencies, so you might get uneven sound in the low end. Stack the two cabinets to minimize lobes--that will effectively make them sound like one larger speaker rather than two distinct ones.

edit: I feel I should also point out that I bet you'll like the sound better with just one speaker. You'll get less distortion from being underpowered and far less sonic interference, which will make the bass smoother throughout the room.

you should be fine as long as you dont turn your amp up all the way and 12's would be louder just because of the difference in size and amount of air that is being distorted but not a hole lot louder

put them into sealed boxes and you will have no problem with them.