Keep in mind I know nothing specific to these machines, I just like the unusual stuff, so I learn as well.
I found this pic on another thread on a different forum. I'm guessing the "interface" is the small board toward the bottom right corner and all of that gets tied into a parrallel port cable shared with one printer on an added LPT card. While the other printer is plugged into what appears to be an onboard LPT port. It's not a great pic, just wondering if it's setup like yours to some extent.
Have you tried just reseating that LPT card, or maybe moving it to a different slot if possible?
Is there a way to test printers in some sort of service menu? Maybe test and then swap printer cables around and see if they both still work if need be. (to test the card/slot they are plugged into so to speak)