not necessary to instance...kurt would have showed how to do it if it was.
Are you selecting from the side view?
Make sure you're not selecting the vertices individually, but draggin the selection handle around the vertices, that way you select the vertices on both sides.

If you are dragging the selection handle, then most likely you have backface culling on, which means that you can only select what's currently facing you. Make sure backface culling is off so that you can select both sides at once. I dont' have maya in front of me right now, so i can't look up where the setting is...perhaps someone else can help? I think it's somewhere under Window->General->Settings/Preferences