Using two subs

How much sonic improvement do you gain by using two subs vs. one? Are there any downsides? What about if they are NOT the same brand?
This subject is highly debated. Check forums. Both configurations can work well. Set-up is key either way. What works well in one room may not work well in another. If you have two subs then start experimenting. Try one sub to start. between speakers, near front wall, side wall, offset, centered, equidistant from listening position. Switch it out with the second sub once you find some spots you like. Next, stack them in the center, try stereo (one by each speaker). Try one up front and one off to the side wall.
The advantage to using two subs is this:

A single subwoofer will give you a unique pattern of reflection-related peaks and dips depending on where you place the sub and where the listening position is. These peaks and dips will be large enough and far enough apart to be audible.

With two subwoofers placed asymmetrically, their respective patterns of peaks and dips will never coincide at any listening position so the net result is smoother bass throughout the room.

You'll need for at least one of the subs (usually the one farthest from the main speakers) to have a 4th order crossover so that you don't hear its location.

No reason at all why you can't use two different subs. That might even result in a smoother overall response.

I use four small subwoofer modules in a system I showed at T.H.E. Show in Las Vegas about a month ago.

One disadvantage would probably be this: For a given total dollar expenditure, one big sub will usually give you deeper and louder bass than two or more smaller subs will.

I went form using one to two subs. The obvious benifit is of course more power and more apeaker. But I really can't explain it. With one sub I had bass. With two subs I have the instruments that are covered by subs. The bass appears to come form my main speakers.It's like they are not here.
Thanks for the responses so far. A little more information about my system and the potential additional sub.

Room:15W x 20L x 8H
Full 5.1 currently: All full range (Klipsch 3-way towers)
Current sub: Klipsch RSW-15 (650watts 15" passive rear firing)

I am thinking about adding a REL Stadium III for the second sub. (200watts 10" ported down firing)

A few things I like about the REL: local purchase, can be integrated easily with my 2 channel setup, and hopefully will strenghten my lows for HT.

One reason I'm doing this is because I know that Klipsch speakers/subs aren't the best at the very low end.

Am I on the right path?

Setup caused the problem you experienced with a single sub. A single sub, properly setup, should disappear as well. But setup may be easier with two subs. Don't know about your subs, but I have mine turned down to a quarter scale at the sub and use the pre attenuator to match the 5.1 levels with white noise and my RS SPL meter. It can generate that felt vibration with a low pedal note or make you feel your chest is compressed with an explosion in a movie. So I doubt adding power is the reason for a second sub.

I went from one revel b15a to two and the difference is FAR from small.
Even though I could not hear where the sub was I could always feel the "pressure" on one side of the room was more. Adding a second sub has evened the pressure as well as aided me to level the bass response out in the lower registers.

I would never go back to one sub having lived with two now.

I run a pair of subs and won't ever go back to single. Once my system is set up as I want, I will set up my subs to carry the LF of my mains and LFE at the same time. I do not hear my subs at all but definitely feel the LF. One day I may even EQ the subs for optimization.
The American way...two is better than one, and bigger is better! :-)

Seriously though, I too use two subs and with the little on-board one band EQ, phase, level matching, and good placement, I have gotten them to blend in seamlessly with my mains.

IMO, this cannot be done by ear, and even with the RS meter it is nearly impossible. Especially setting the phase. I now use ETF measurement software, but the Velodyne SMS-1 very effectively uses a test tone sweep and microphone. Even if you don't EQ them, this way of measuring them and graphically displaying the results really helps the set-up process.

I cannot imagine how people do this any other way.