what can i expect from a 2nd sub?

I currently have a single Infinity Perfect 12 (single 4 ohm voice coil) in a 1 cubic foot sealed enclosure. I am running it with an MTX amp that puts out 210 watts at 4 ohms and 420 watts into 2 ohms, so I am only using half the amps possible output with the sub. I am planning on purchasing a 2nd Perfect 12 and putting both subs into a dual sealed chamber box so that each sub will get 210 watts.

My question is, what can I expect as far as difference in sound from adding the 2nd sub? I am doubling my current setup in both the driver and the wattage...should I expect twice the bass output? Just looking for opinions on the subject.

I would say go for it.  Right now you have a system that is neither SQ or SPL, but it looks like you are tending toward the SQ way of doing things.  However, the power applied (150 watts at 12.5 volts) to that sub in a 1 ft box is meager.  Better results would be had if the box were 1.5 ft or a little more.  And, the amp should perform similarly into either a 4 ohm or 2 ohm load.

And so I say do it because you now will have a chance to build a new box.  WinISD has this sub in its database and shows a nice setup with two subs in a common sealed chamber of total 3 cu ft net.  You will still be tending toward the SQ way of life, and near-future upgrades of your main speakers will further you along that path.

And did you read THIS THREAD?