In reply to:
and the 350 and 500 are in the same size cabinet
The 350 is not as long as the 500, right? But the 350 would certainly stack on top fo the 500, just not the other way around.
I have 2 500's that are not co-located. I put one in the front and one in the back of the room. I did this to try to even out the response in the room. I haven't felt the need to co-locate as I feel I get plenty of volume from these 2 beasts. In fact, I have to keep the volume knobs and receiver sub adjustment pretty low just to keep them calibrated with the rest. My room is under 4000 cu. feet however.
In a 7600+ cu. foot room I can imagine you would want the extra SPL output. A 2nd sub will give you 3dB more output, or 6 dB if it's stacked on the other, or at the least next to each other in a corner.
If it were me (YMMV), I'd probably want the 600 in a corner over two other subs stacked for 2 reasons. 1) I think the 600 would look nicer in the room than 2 smaller subs stacked (IMO), and 2) It's cooler to have the EP600. Afterall it is Axiom's flagship subwoofer. You won't have to suffer from further upgrade-itis.
The downside to a single 600 as Craig mentioned, is that the 2 stacked subs will give you roughly 4 dB more output (115 vs. 111 dB from the 600). But for me the cleaner look of the single 600, and simplicity of having only one set of sub controls to fiddle I'd go with the 600. Not to mention you get a little bit more extension with the 600 (down to 17 from 20 Hz).
Best of luck on making the decision. A tough one for sure. But remember, either way the system will rock.