I do not recommend combining two different models of subwoofers for use with the same audio signal. The phase shift of subs can be pretty extreme at the bottom end of their operational range, so if you use two disparate subwoofer models, they will shift away from being in perfect phase, especially at the lowest frequencies, and thus start cancelling out each other's output signals. If you combine a vented sub with a sealed sub, this effect is vastly increased and you could get a result where some frequencies are almost entirely cancelled out in the room and you'll get significantly less bass.
Many times I have been asked to help someone "tune" their subwoofers which were not matching identical models, and the results are only very, very rarely better than a single sub at the lowest frequencies.
I recommend either buying another identical model as you currently have, or sell your current model and buy two subs of a different model.