Dual VS18.1: 130dB from 20-80Hz
Dual SS18.1: 131dB from 40-100Hz
Single SS18.2: 131dB from 40Hz-100Hz
So I think dual SS18.1 for $1K is a good deal.
Yes, for 2 subs, I would leave the XLS1500 in Stereo mode for 525 Watts RMS x 2ch.
I think that is enough, although I think a lot of people would recommend getting two XLS1500 for 1550 Watts x 2ch.