Start by taking one of the plotters out of the equation and then figure out what port your cable to the remaining plotter is attached to. Once that is done hooking the other plotter up will be pretty straightforward.
Your plotters, BTW, are designed to run from Comm Ports. Do you have two Comm Ports or are you confusing LPT Ports with Comm Ports? Are you using serial cables (correct) or are you using printer cables? What version of Omega are you running and does it have an LPT or USB dongle? Have you setup the Rainbow Sentinel Configuration properly?
When testing, make sure you have your plotters set to online.