Best setup for two subwoofers.

Bottom line here is you have so many variables in play here, it's gunna be nearly impossible for anyone to discern practical application here. I mean,it's all relative here, and any setup needs to balance out, with all issues addressed, and all variables qualified and considered, relative to "truth".
How big is this space? Can 2 subs even work in his room? I haven't seen it. How's the construction? What's the layout? How many seating positions in play? What's the lifestyle of the listener(s)?
Not considering all of the above -even knowing none of the variables - my simple answer is that you're likely going to run into fewer issues (phase, frequency response, comb-filtering, localization,etc ), and get the best integration and bass/sound quality running multiple subs (ok, in this case, two) if you try and keep a set of subs up near the front/center of the room or setup, near the center channel speaker. I would set them side by side in the middle of the front of the room (or stack 'em just off room center axis), along front wall, for a likley rock solid integration with a multi-channel system, and you'll have great off-axis phase integration with the main speakers, (if adjusted properly relative to mains/center), good integration, blending, etc, and you'll properly deal with modes 1 & 2 in the bass response (smoothing out the bottom). Anything else, and today's superb digital EQ's/DSP integrated into many AV recievers and pre-pro's is taking care of smoothing out things even further, fixing phase, level matching, response, yada, yada.
While you can put subs all around the room, in various places - with benefits one way or another - you'll be dealing with phase issues from seat to seating position and sub to speakers, challenging speaker to subwoofer integration from all seatting positions, distracting localization of upper freq's, obtrusive subwoofer speakers cluttering your space, etc.
Bottom line, keep it simple. And, of course, I know nothing of your room and associated equip.
Stick with what I'm advocating, and it'll likely fit very well, regardless of what your seutp is, and all the variables of which I know nothing of.

When all is said and done, your goals here should encompase these important points - if performance is at the forefront of your target: even base response from the woofers- with no major holes (peaks avoided and knocked down, if EQ'd), proper phase and near seemless integration between speakers and subwoofers from all your seating positions (there's always going to be some level of compromise, all things considered - but, obviously, you're striving for the least amount of compromises), proper level matching between woofers and speakers, and try and avoid frequency filtering by placing subs distances where you'll be canceling a frequency that's in a critically covered spot by the subs (1/4 wavelength of the distance betweeen them)
After that, you're only concern is the lifestyle choices with having gear all over your floor! Subs can look awefully unsightly stacked all over the house. Er at least so my girl says...

I can give you a more practical response. I did what you were asking. I used to run a RT 10d with a Rel sub. To me it was not sound I was looking for. I went with a pair of JL 112's which sound great. A pair of good subs really did it for me. My advise is to get a better sub see how you like it. Then get another matching sub if you are not completely satisfied. Setup is important but a good quality sub is going to make a huge difference. There are a lot of great subs out there. Once you hear them you will see where the klipsch comes up short.

"Setup is important but a good quality sub is going to make a huge difference" (Docrobbi)

My experience professionally, and decades at this, is just the opposite, actually. Just to consider. I would put this comment as "A good quality sub is important, but SETUP is going to make a huge difference!"
I've simply heard way way too systems over the years, that used some otherwise pretty exotic high end subs/gear in them, that sounded not only aweful, but performed disasterously, because they weren't set up properlyAnd I've sold some of the most expensive subs in the world over the years! - so I feel qualified to say this.
So, yes, I put proper setup, room acoustics, equipment matching, engineering, and calibration - even BALANCE - over quality of gear in this case. And with subs, it probabaly even more true here. Setup and integration is everything when it comes to subs/bass

If you have one sub to fill in deep bass for front speakers and one for LFE it can be okey to run different brands. But if you want to run both subs for LFE they should be of same brand and ideally also exact same model.