No, only one. They do not like each other since they occupy the same ecological niche.
Hey sorry to keep bugging you, but I made a thread to see which Goby to get (diamond vs watchman) and others are saying that they will be fine together and should get one of each? http://www.reefcentral.com/forums/showthread.php?t=2305510
Based on this thread alone I feel as though you're the person to ask, I just wanted clarification as I may have worded the question poorly (as I did not mean multiple neon gobies and multiple diamond gobies, but a single neon and a single diamond in the same tank).