Mixing subs is "risky" (risk of bad sound) and generally not recommended if you don't know what you're doing - but it's not necessarily impossible.
The problem is different sub designs behave differently, especially phase-wise, meaning that different frequencies may come out at different times/in different phases of their cycle. And this varies from model to model/design to design.
If the two models in question are too much out of phase at certain frequencies, they will work against each other at those frequencies and cancel each other out, resulting in poorer performance and uneven frequency response.
Things you can do in your situation:
You won't know before you try it, so if possible get the subs together and see; For the test just use one of each for simplicity.
Set them up side by side, play one at a time, switching back and forth and adjust their levels so they match.
Then play one and then while playing switch on the signal to the other one and notice what happens. Ideally, it should sound the same, just louder (6 dB to be exact).
If it sounds good, it is good. But you have to play several different tracks, preferably something with many different bass notes, not just one kick drum or loop or something. Notice if some notes are way louder than others.
You might also want to take a more analytical approach: Take a sine wave generator (download one on a smart phone f.inst.) and play various frequencies in the subs' operating range (probably something like 40 to 100 Hz). Try many different tones, playing one sub and then switching the other on and off. It should get louder by an equal amount on all different tones/frequencies. (Caution: you don't have to do this test loud at all - it's probably easier to hear at lower levels, and especially with continuous sine waves you DON'T want to play loud at all, otherwise you'll burn your voice coils!)
If all tones (and all music tracks) consistently do not add up nicely between the two subs, there are a few things you can try: Flip the polarity of one of the subs (they probably have a switch for this at the back) and see what that does (if they behave well to begin with, this should make them behave badly and vice versa...to some degree). Other than that you can try delay one sub, if you have a device that can do this (DSP of some kind). If not, you could try to delay one sub by moving it backwards/away from the listening position/area. You might be able to find a spot where the various frequencies add up better, also try this in conjunction with flipping the polarity.
But: if some tones add up well while others seem to cancel out rather than add (and this is not unlikely at all) - that's where it gets messy. Then you need to have some advanced DSP to correct, and it may not be possible to really get it right even then.
So in conclusion: Mixing subs is something you don't do if you can avoid it (= if you have enough rig/budget), but you might as well try, if you're not doing sound for royalty and there's not really any alternative...
Finally, an idea: If they don't play well together and it's a large area you need to cover, you could set up two systems - one at each end of the place. It'll make holes some places on the dance floor, but they might not be too bad. It's not ideal, but it might be something - just thought I'd throw it in there.