the CHEAPEST way is to simply move the plug from the mac to the pc whenever you want to use it. But that is impractical. You didn't mention what type of monitors these are, but if they are regular VGA input monitors, a standard KVM switch would work. They make USB ones that would also let your keyboard and mouse switch over to control the PC. If you have an ADC monitor, you may need an ADC-DVI adapter, which is costly and a switch box. Someone does make a switchbox that accepts one ADC monitor, and lets 2 DVI computers control it.