take the postive side of both speaker wires and twist em together and put them in the positive speaker terminal on the amp, and take the negative side of both speakers and twist em together and put them in the negative speaker terminal on the amp. You're mono amp will "think" you only have 1 sub, but both will recieve signals.

and as mentioned, subs do NOT need a power/ground connection, nor is it even possible. all they need are the positive and negative speaker wires.