cscmpt, yes it oughtta run two machines, you might run into some "fun" with different volume levels going into the left and right, depending on the two pin's amp sections power output. you might want or need high (speaker power) to line level adapters, then run signals into the RCA's. high to line adapters with trim pots (gain) preferred. you might be able to do similar with low value pots inline to the high inputs. my own preference is just replacing a machine's woofer with one that is a lot more efficient for thump (hello MCM?) without a secondary outside it, or if I really feel the need, put an amp in there (kinda runs "off topic" I guess).

that idea:
stereo left right (backbox) into a high to line level adapter, into a little 2.1 ch stereo amp (like Lepai) and use the amp to feed the speakers. gives volume-bass-treble to backbox and separate volume for woofer that's already low pass filtered.. (is prettymuch exactly what Spooky did with AMH). it'd basically render a machine's audio section to just being a cool running preamp, with a more "disposable" but more powerful amp driving the speakers themselves. it'd be good for lesser efficient higher fidelity (like 4" car coaxial) speakers in the backbox, for having more power (at whim) to drive them with. little Lepai LP-838 is going to want 12vdc 4-5 amps, their 168HA 12v 8-10 amps. for about the price of one external sub, you can put an amp into 2-3 games pretty easily. no concerns about crosstalk or volume levels that might happen with a shared sub, or wires running to a box on the floor etc. not really knocking a separate powered sub, this idea could also cause a lot of rattling glass etc that an external wouldn't.