Well, there are probably a few things to consider before doing this - in theory there is nothing WRONG with mixing subs, more a matter of your personal preferences and tastes.
1. Make sure that your A/V receiver can handle the load placed on it with the additional sub - this includes, the impedance rating of the subs combined. Are both subs powered by their own built in amplifier? Does your current receiver have 2 subwoofer outputs, if not, how will the 2nd sub get a signal - this is important to avoid overloading your receiver.
2. Crossover frequencies for both subs will play a part in the final experience/result you get
3. Power handling, if 1 sub is substantially more powerful than the other, you may not necessarily gain a huge increase in SPL
4. You could use 1 sub for mid-bass and the other for lower end, allowing more headroom for each sub, and the overall quality of your bass could improve.
That said, you won't know until you try it.