I have the Colt Rail gun and RIA 2011 Tactical/full rail. IMO, a light mounted on a handgun is ideal for HD. One hand operates light switches, opens doors, picks up kids, etc and the other has the gun and light in one. The light can be pointed and lit on the ground to see everything in a room. It does not have to be pointed at someone/something to see them/it.
Now a second handheld is always ideal as well. As for carry use, I'd use one without a rail.