If you heat one side of your fingers, the other side naturally heats up as well. With my grips on 50-75% my entire hand, both sides, are toasty.
Well, depends on how cold it is and how fast you're going.
We were riding through the desert in the SW US, early morning during the wintertime - temps were about -15°C and we were going ~120 km/h.
I had both the heated grips and heated gloves cranked to the max and during that ride I thought the gloves had failed because my hands were freezing.
Anyway, I turned off the heat troller, since I thought the gloves were conked out. I couldn't believe that my hands could get any colder, but instantly, all my fingers turned into ice tentacles with only the grips on.
Turns out the gloves were working. They were just ineffective at that speed and temperature...
Anyhow, on another note, the best piece of kit I have ever bought for cold weather riding was my FiredUp heated vest that was on a group buy here a few years back. It's incredible how much keeping your core warm helps keep your extremities warm as well.
I had just gotten my heated liner and was testing it out in early fall. Temps weren't too cold, about 10°C, and I had that thing cranked. It raised my core temperature up so quickly that I got nauseous from the heat. Got dizzy, had to pull over and almost threw up in my helmet.
Turns out there's such a thing as too warm, too fast...
I don't normally wear 'lectrics until it's 0°C or lower.