Getting two lesser subs only gains you 3 dB of volume. If the sub can go low, adding a second identical one won't help. If the sub doesn't sound good, adding a second one won't help. Also keep in mind that placement for two subs is more complicated since you don't want the bass to cancel out.
I'd get one of the better sub rather than two of the lesser sub.
I got the pb12-isd/2, man this sub is awesome. I haven't properly calibrated because I'm waiting for my house to be built. But from just testing where I'm staying right now, it shook the house and a light cover in bathroom fell down, and I didn't really have it turned up. This sub rocks! It may be big for some, but then again, size does matter in this sense. Glad I bought it.