Here's my first posted REW graph of my dual EX's. I have a bunch of graphs, but this represents the final resting place for the subs. One box is front right and the other left rear. Front sub with built-in crossover off (just using receiver 100Hz crossover) and the rear also using sub crossover set to ~80 Hz. The two lines show something I found interesting after moving the boxes around and playing with crossover ranges, etc; The phase on the rear is set the same as the front with the blue line, whereas the red line shows the phase set opposite with the rear, and seems to yield better response. Also, these measurements are taken from the primary seating position (my chair). Elsewhere in the room, this setup also provided the best results...something I was aiming for with the dual setup.

Here's another comparison...
Red: final resting place, front right, rear left, front crossover off phase normal, rear crossover on, phase 180.

Blue: both subs up front near corners, crossovers off, phase set the same on each. This was the best looking front location settings versus other crossover and phase setting with both up front. Still not as good as the diagonal opposite locations.

Here's a comparison of duals versus just one or the other...
Red: final dual setup
Blue: front right sub only, crossover on
Green: left rear sub only, crossover on

Ok, one more graph for now...
This is a comparison of two different seating locations (chair in middle of room vs. loveseat near the wall right next to rear sub)
Chair: red line
Loveseat: green line - a little more pressure due to its close proximity to the rear sub I'd suppose or near a wall

Room picture prior to subs arriving...

Nice graphs. To me, those red lines do look better. How big is your room? So you tried both subs up front and both in the back of the room and they worked better split up? Just wondering.

I tried both up front, not both in back...I really don't like feeling a sub behind me too much, especially for music. That's why I tried both up front with all combinations of crossovers, phase, etc. but that just didn't yield as good a response, so diagonal it is! My room is 15wx17dx10h and opens up to a double tall great room.

It's good that you experimented so much. I think people like subs co-located so they get the biggest bump in SPL. But for you, the other way works better. That is a pretty good response you have in a 2500+cf room that is still open to another room.

good info on duals

multiple subs will always allow smoother inroom response, as was shown in the often quoted Harmen paper, as well as in Dr. Geddes work. However, one thing you seem to not be keen on, but was explicitly mentioned in both of those research works on the topic, is placing the subwoofers away from the front speakers. Dr. Geddes promotes the use of 3 or more sources, Harmen promoted 4, but since you only have 2, I would argue that opposing sidewalls might be the best location.

The wavelengths are so large below even 300 hz that in your room, it won't really matter where you place them (assuming they are still in the same room) they will couple to increase the maximum spl you could achieve. Your goal should be to get the smoothest response, and in your own graphs, it shows very clearly, and consistent with the research, that one subwoofer suffers from very bad room mode problems, and two nicely fixes much of that. If you placed those subwoofers right on top of each other, they would largely act as a single bass source, you would still have the room mode problems, which is why people buying or building subs with lots of drivers or placing them all in one corner are largely shooting themselves in the foot. However placing them all upfront with the speakers is only slightly better than having them all in one corner, or only having one sub. You have done lots of good work so far and show just how good a response improvement you can get with two subs. I think further improvements might be had though if you were willing to try more flexible placements, such as side wall locations.

Since your main speakers look to be of decent enough size, I would consider running them full range with your processors crossover set quite high, say 120hz or more, and then playing around with each subs crossover point to see if further improvements can't be made. This will give you 4 bass sources. Also play around with one sub in a corner with another in a mid wall location.

Consistent with Dr. Geddes view of multiple subs with different Q's, you could try sealing the ports on one of the subs. It won't hurt anything, no worries there, the driver will just have a response as if it had been placed in a sealed cabinet. Less output around the tuning frequency, more output below the tuning frequency, and maybe a little better excursion control. The amp probably has a highpass filter for protection, so you may not get any more output below the tuning frequency, but you still are changing the Q and thus exciting the modes different with each bass source.