OP, for pair of subs, use the supplied high level Neutrik Speakon 3-lead cable as below,
Twist red and yellow wires together of LEFT REL sub and connect them to the L channel positive binding post. And connect black wire of left REL sub to the L channel negative binding post. Repeat these steps for your RIGHT REL sub / right channel.
In some instances, twisting red and yellow wires together may result in exceptionally high gain (output) from the RELs. If this is the case, please remove either the red or yellow wire from the twisted pair and use electrical tape to mask the loose wire. This will reduce output by half and restore a natural dynamic.