Getting them to play nicely together is the problem. Short answer is yes, you can, but there are a lot of complications introduced. Because of the way each operates, it seems one of the biggest issues is the introduction of frequency cancellations beyond standard room nulls.
I would say, in general, get multiple ULS 15... you will have the benefit of cabin gain on your side to enforce the lows, plus the mid bass strength, especially if your room isn't too large to minimize the benefits of that...
Or get multiple VTF 3 or 15s that will give you both quite readily. Neither of those options is likely to do you wrong, and will make the set-up and implementation much easier overall.
Mixing and matching different subs starts getting into the territory of using DSPs to tune them and help minimize the differences. Also, careful selection of subs helps, choosing two subs that have similar responses, for example. I am uncertain if mixing Sealed and Ported options is a good idea. I am inclined to think not, if I remember correctly the previous conversations where this has been broached.
