Yes, I'm in agreement with pretty much everything you've been told here so far. I'll just add a couple more examples:
- If you have multiple subwoofers, what you are going to hear is basically the "worst" subwoofer in your setup "pulling down" the quality of your bass, not your "best" subwoofer "pulling up" the other subs.
If one sub has worse transient response - with high group delay, lots of overhang, or just generally "slow" sounding response - then that's what you're going to hear. If one sub has higher distortion, then that's what you're going to hear - and this is especially important if one sub plays lower than the other, because the sub that cannot play as low might be attempting to produce frequencies that are below what it can reach without going into high distortion.
It really is a case of "the weakest link" in the chain.
As Granteed was saying, if the sub that cannot play as low has a very steep filter on its low end - so that it does not even try to play lower than it can and prevents it from producing high distortion, then things are not so bad on just the extension front. But you still have to deal with the transient response - where what you will hear will be the "slower" of your two subs. And when you turn the volume way up, which ever sub starts to distort first - that's the one you're going to hear, so at all times, you are limited by your "worst" sub in those departments.
It isn't always the case, but the likelihood is that a little 10" is going to distort at a lower volume than your larger subs. You'll have gained around 3dB of output by virtue of having two subs playing, but you'll be limited to the maximum distortion free output of the little 10" sub, which means you've likely not gained any real headroom by adding it.
The 10" almost certainly won't play as low. Now it's a question of whether it has a very steep roll off on its bottom end so that it doesn't start distorting and compressing trying to match the extension of your larger sub.
And while some 10" subs might be "faster" and have better transient response, you're not going to hear it. If the 10" has better transient response than your larger sub, you're still only going to hear the transient response of the larger, "slower" sub". The "slower" sub will always mask the "faster" sub.
So essentially, you haven't gained anything! As Granteed mentioned, provided you place your subs properly, you will gain smoother frequency response at more seats for the frequencies where the two subs are both playing - so that's your only gain. If that's the only issue you're trying to improve, then ok. But you're not gaining anything else, so it isn't really the best use of your money, IMO.
As was mentioned, if you feel as though you are missing some mid-bass "slam", consider something like the MBM-12. As timoteo pointed out, that is exactly what it is designed for. You feed both the MBM-12 and your large, deep-playing sub the exact same subwoofer signal. You set your deep-playing sub's internal cross-over to 50Hz, meaning that it will filter out everything above 50Hz with a 24dB/octave slope. The MBM-12 already has filters in place to filter out everything below 50Hz at 24dB/octave, so you've created a 4th order Linkwitz-Riley cross-over at 50Hz between the two subs. The cross-over between the MBM-12 and your speakers is handled as normal by your receiver. So you've basically just made your subwoofer into a two-way system - very much like a two-way speaker with a tweeter and mid-woofer. The MBM-12 is handling the "high" portion of the bass and then crossing over with a 4th order slope to the "low" portion of the bass being handled by your larger, deep-playing sub.
Now, with the MBM-12 system, you're not gaining anything in the way of more even frequency response at multiple seats. You're essentially still just using one subwoofer.
So if you "want it all", really the only way is to get a second subwoofer with identical performance to the first. Typically, that just means getting a second identical subwoofer! Or replacing what you already have with dual subs