Arg Clin - it
should work given that X-750 is about as reliable as they come. In
tests, X-750 is capable of pushing almost 800 watts on the +12V alone (albeit in short bursts, not recommended continuously). In your case, +12V load will be about
250W * 2 for the GPU's plus up to 100W for the rest, so 600W. Power spikes possible up to 700W. Since these numbers are still well below the PSU's rated maximum +12V load, there's no reason it shouldn't work - but I'm not 100% certain on how the unit will deal with such loads in the long term.
The only real problem you're facing in the short term is the fan which will certainly not remain quiet at load, but then again, I don't think 290X Crossfire is going to be quiet anyway.
Personally I wouldn't risk wearing it out, I'd rather upgrade to a single high end GPU like 980 Ti or replace the unit with 850W or 1000W, whatever's a good buy in your local market. Another problem I'm seeing is the Define R3 case- not exactly equipped to deal with 600 watts of heat dissipation, and pretty hard to custom configure it to do so without using ridiculously high RPM fans. So you would probably also need a case upgrade for 290X Crossfire (e.g. Phanteks Enthoo Pro), whereas a single 980 Ti wouldn't consume any more power than your current 290X.