You can use more than one sub at at time in any setup, however there are some steps you'd have to perform to make sure they don't cancel each other out at certain frequencies and as you mentioned, ITistic, the aesthetics of two subs (as well as the size they'd take up) may be a consideration.
Honestly, I don't think you'd even need to integrate the KSW-10 with a PB10 or 25-31 PCi, they would likely overpower it and overcoming some of the placement/cancellation issues of two subs might not be worth it.
Extension is essentially how low the sub can reach without the THD becoming a substantial factor. The deeper the extension (lower Hz number for frequency response), the deeper/lower the bass you'll get... -TD
edit: The PB10 would likely be fine for a room that size, but a 25-31 PCi would be even better...