identical subs sound different

The only way that I know of that sound level becomes reduced when you play two subs vs one is that they are out of phase with each other.  If each sub sounds right by itself, you do not have defective equipment, you have a defect in the installation.  "Phase" means either exactly what aggie altima said: when one speaker is moving out the other is moving in and they are cancelling each other, OR if they are both electrically in phase, it is possible the sound waves are cancelling each other due to speaker position, car interior geometry, or any number of other factors.  Here's what I suggest you try:

Ensure the speakers are electrically in phase starting by ensuring they are connected correctly + to + and - to -.  Trace the wires by hand from the speaker terminals to the amp.  Check your feed to the amp.  Is it running off a pre-out from your deck or are you using high level signals and a LOC?  Check you amp and see if it has a set of "phase" switches and if so, make sure they are set to the same position on BOTH channels.  Make sure you are sending a mono signal to both channels of the amp and that it is the same signal.

If all this is OK and you still notice cancellation, try moving the subs around.  Are they in the same enclosure?  Sealed, vented, bandpass, what kind?  Try re-positioning it, point it in the oposite direction, make sure there is the same distance from both drivers to the nearest thing in the car directly in front of them, make sure the ports are the same length and not obstructed either externally or inside the box, etc.  If they are in separate enclosures, make sure they are not pointing at each other in addition to the above.

Good luck!