If both pc's use the same type (DDR2 or DDR3) and support the speed (1333, 1600, etc), then yes you can. However, if you use single stick in each computer, you will not get the benefit of dual channel capability.
Look for a 4x4 set of ram and then use 2x4 in each rig. Or, if the current ram in both pc's is identical, move the ram from one pc to the other and get a 2x4 for the now empty machine.
If you tell us what motherboards/cpus/ram you currently have, we can give you better information.
Mark