That is a possiblity; I looked at your tank size before recommending 2 males, and 40gal pretty heavily planted sounded to me like enough room for one to get away from the other... I've never actually kept dwarf rainbows, though, so you might want to do a little more HW on that one to be on the safe side. Maybe the Fish Profiles on here will give you some more insight, or
www.FishBase.org may link you to another site with more info yet.