I've done the series thing on a War Gods cab before. the amplifier on my MK2 didn't like it too much after awhile.
I would thus go parallel instead. take the pair of wires to the left speaker and cut the quick disconnects off. take 2 wires for + and 2 wires for - and splice them in, then run those to the 2 speakers.
be mindful that some Williams/Midway games that use volume pots positively hate that stereo JAMMA harness and you will have no sound at all. if anyone knows the logic and reasoning behind why this is, I'm massively interested.