How do you hook up a pair of REL T5x subs to a class d amplifier?

Look in the comments section of this video: https://www.youtube.com/watch?v=cV_0G3GdPxQ

See REL's reply to the question from mark wilson. (some others also posted similar questions and the replies were also similar). Basically leave the black wires disconnected (if you get hum, some remedies are described). If it isn't clear, I suggest that you contact REL customer service.

Since the Bel Canto Evo 200.2 is a Class D amplifier, the black wires for your subwoofers' High Level cables should not be connected to the negative terminals on the amplifier. Instead they should be left disconnected to start, and if hum occurs from the subwoofers then you can either connect the black wires to a chassis ground screw, or you can run a pair of RCA cables from your preamp to the .1/LFE input on each subwoofer. The red and yellow wires of the High Level cables will be connected to the positive output terminals on the amplifier, as outlined in this video.

Yes, thank you, I found that. My question has to do with hooking up a pair of subwoofers, do you hook up both of the speakon cables in parallel to the amplifier terminals?

you can run a pair of RCA cables from your preamp to the .1/LFE input on each subwoofer. 

That would be my choice. 

I want to use the high level speakon cables.

Yes, you can run wire from Sub 1 to Sub 2. That assumes your ground situation works. Good luck with that. I’ve never had the “leave the black wire off” solution work on differential (in your case digital) amps. You can try the other methods, they may work. I had the RCA cable to LFE for ground work once. You will most likely wind up using pre outs-RCA. Honestly, it doesn’t really make much, if any, real difference.