I have a remote with a single probe....do I need a dual probe thermometer? I thought that the rheostat controller would be close enough for ambient?
Do yourself a favor and never, ever measure the box temp unless you have an extremely good reason to believe the smoker is malfunctioning. It will drive you nuts and provide you no value at all. The box temp is going to swing +/- 30 degrees during the smoke, but over the long haul it all works out with no ill effects.
Do you need a dual probe meter? Probably not need, but I do find that I use both probes quite frequently. If I have a large brisket I'll cut it in half and probe both the flat and the point separately. I frequently smoke butts two at a time and will probe both. Even for a Turkey I'll probe two locations so that I have a sanity check and backup.