There's probably no reason whatsoever to run TWO cables to the JBL. It is possible that it doesn't even need to have both inputs. My SVS has both inputs, but I only need to use one of them.

I'd split the signal at the receiver with a Y.

Run one leg (mono) to each sub.

If your JBL for some reason doesn't like one input, Y it again back to two.

bibere usque ad hilaritatem