What you don't want to do when mixing subwoofers is to use one that is substantially more powerful than the other, but the vtf2 and 3 are pretty close in performance, I think they can be used together with no problems. The VTF3's big advantage is in the lowest frequencies at the bottom of their tuning point. In the end I would rather have a VTF2 and a VTF3 in one system rather than just two VTF2s, the two systems will be a lot alike except the one with the VTF3 will have a bit stronger deep bass.