Two subs or not Two subs?

I have been contemplating adding a Rel T-3 Or two.To my vinyl only 2 channel set up.

Proac responce 2
TAD Hibachi-II Mono Blocks
PS Audio GCPH
Pro-Ject 2 Xperience with an Ortofon HMC 20 cartridge

My room is 20'L x 15'W x 10'H and the speakers are on the short wall.
First are two subs better than one. what are the benefits of having two vs. one. Can I get by with one. Thank You