Wiring 2 subwoofers in a single system

As long as you start with the volume controls low on everything, you won't blow anything up by experimenting. If it doesn't hum loudly or anything when you power it on, then start upping the volume starting with the farthest out devices. The only thing I can think of that might be a concern is the new sub is outputting full-power signals to the front speakers at whatever volume the whole system's set to, but I wouldn't expect the cables to be compatible if that were the case.