AG, I ran dual subs for a long time and liked what I got. Placement is less critical(I had cheepies) and the flexibility is fun.
I would recommend the same sub again(if you like the first one).
Using dissimilar subs can be tricky, as cancellation of waves comes into play, and that can be hard to track down. Twin subs can do the same, but alternate placement can overcome the cancelation. The issue then becomes timing, as the waves are disparate, and the sound can get 'muddy'. Placement of twins is kind of fun, so long as you find the right spot eventually.
ANother member, David Pannell has dissimilar subs, and he overcame these issues with some dogged determination. Perhaps DP will catch on here for you.
Srinivas, I solved that with a Velodyne sub! hahaha. I never could place the bass where I wanted it, but my Psb speakers are not bass-oriented. A bass heavy(for me, well balanced recording would take care of it. John Mellencamp's uh-huh would fill the room, yet other recordings would not. I got tired of moving the speakers, so plunked the sub in the corner.