You could but you'd have to be right next to them. I've had a situation where the battery in my Quarq died on a group ride and my Garmin kept trying to pair with other riders' PMs. But as soon as they get more than a bike length or so away, you'd lose the signal. It's ultra low power, which is why you can power a PM with a watch battery for several months.
And besides, all a power meter measures is power (hence the name). It doesn't have any way of measuring how much it hurts a particular rider to produce that power.
I can't speak for anyone else, but in a race I don't even look at power. I capture it for later, but in a race all I care about is whether I can hold a wheel or not, whether I feel fresh enough to launch an attack or not, etc.
Last edited by caloso; 09-16-20 at 12:23 PM.