Some sub woofer amplifiers have a set of hi level or speaker wire level inputs. These are designed to take the speaker cables from another amplifier and use this hi level signal to drive the sub.
If your sub amp has these you could take a second set of speaker wires from your second speaker output on your stereo amp (probably marked speakers B or something)and connectup to these hi level(or speaker level) inputs. These will have screw type fittings or something suitable to accept speaker cable.
If your sub amp ONLY has rca inputs then try splitting the rca as described earlier,
