A single roller watch will have one plate holding the roller jewel. There is a notch in the plate matching the position of the jewel, and a vertical pin on the pallet fork that prevents the pallet fork moving across to the opposite banking unless the jewel is in the fork.
A double roller watch has two circular plates, one holding the roller jewel and the other, smaller plate having the notch. The safety pin (sometimes called a safety dart) is horizontal and projects underneath the fork. It also prevents the fork from crossing to the opposite banking unless the jewel is in the fork.
A double roller is more "positive" in the engineering sense in that the operating distances are smaller and the parts are stiffer, which should result in less occasions where the watch goes out of action from the fork being on the wrong side of the banking when the roller jewel comes around.
I have never seen a single roller wrist watch excpet possibly an old pin lever one I have floating around that I cannot take apart. Might be some out there, but I don't know of any examples. I would assume any wristwatch with a jeweled train would have a double roller, and even a 7 jewel Caravelle I cleaned recently does.