Considering adding Power to our Tandem. would this be a useable metric from a Left Only crank based meter? or will the other rider’s input cause a issue with accurate measure?
It should measure power just fine, in most cases. You will be seeing the power entering that specific crank, by the one rider.
It might lead to some interesting results in odd cases, like the PM rider not pushing, but actually “back pedaling” in opposition to the other rider. No idea if the PM will report power in the event of a negative" force in the opposite direction to the spinning cadence (if it even can tell the difference).
Would be fun to see the numbers if you get a PowerTap hub to measure the total for both of you
That’s my concern, I’m sure it will measure power, just don’t know how ‘useful’ the info would be as the other rider can certainly have an impact on it.
Depends on what you want to use the reading for. If it’s to measure the effort of one of the riders, it will work fine (with the caveat indicated by @mcneese.chad). If it’s to measure the performance of the team, it will be useless, a hub-based PM would be needed (and probably impossible to find that can be built into a tandem wheel) - or 2 sets of pedal or crank PMs, maybe.