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.