This is the one thing about Maya that I have problems with - I think there should be a modifier in Maya that does that automatically. Cinema 4d has one that works well. I think it is called "symmetry"? It works well because it welds the two halves together while you are modeling and when you are through you make it "editable" (something like that). It replaces numerous steps that you have to do in Maya.
If I knew how to write mel, I would make a plug-in that does that. but I don’t