Two Subwoofer set-up in home - do they need to match?

I'm building a home theater in a large room (6000 cubic ft). I own an Axiom subwoofer. I'm going to buy a more powerful subwoofer (or two). Can I have one great subwoofer (that I will buy), supplemented by the one I already own? Or for true performance, am I substantially better off investing in 2 new subwoofers?

The main problem with multiple speakers that don't match is that there is a chance that almost all of the sound will come from only one of the speakers. The speaker that is more sensitive (efficient) will put out most of the sound. It is often hard to know the speaker's sensitivity (sound pressure level at 1 watt input–it is not usually printed in specifications, except for very high end speakers). If you can get that figure for both of the speakers you want to use (call the manufacturers if you have to), then if the numbers are within 50% of each other, you are probably ok. If the speakers have different impedances, that also complicates the issue. However, many subwoofers are powered these days, so:

If the speakers are powered, and there is a level control on the speakers, then you can adjust the output volume of each one separately so they match. It depends on your budget and the design of your space.

One thing to keep in mind is that most properly designed theaters have a degree of acoustic absorption and bass trapping (if not...that should be your FIRST priority). This smooths the in-room response but does tend to soak up a good amount of energy. The same sub that may fill a 6000 ft3 untreated space may have issues in a 6000 ft3 treated space.