The only reason for 2 subs is if you don't have space for 3 or 4, but you do. The poster who claims you absolutely don't need subs with these speakers is unfortunately ill informed and has obviously never heard a good multi sub set up. I suggest he read Geddes, Welti or Toole
The advice to look at Rythmik or GR Research is solid, a servo system that IMO outperforms most. @hilde45 mentioned variable phase which is essential for truly dialing in the subs which will exclude most of the REL range. Another drawback of REL is their use of passive radiators which behave in a similar manner to a ported speaker which I also suggest you avoid. A port is another bass source but unfortunately it is frequency invariant and therefore can't be tuned. Home Theatre guys like them but my recommendation is to just stay away for music. I own a REL and a small SVS SB1000 and it handily outperforms the REL. Sorry REL owners, I gotta tell it 
To optimise the rig you're advised to measure your installation using REW (free download) I use OmniMic V2 to measure but it is now discontinued and replaced with an improved version available from Parts Express for $599, I think.
The crawl method to locate the best spot for the subs is poor advice although popular. What you will be locating is the loudest bass and that will be from one of the room's many peaks but what you won't/can't hear is the deep null nearby. It's a null, a black hole for music so there is nothing to hear, however measurement will reveal all peaks and nulls and juggling the parameters will flatten this out to a smooth response. The variable phase function will allow more freedom in placement.