I get the quantity argument, but how does a team manage the bandwidth in a manner that gives both QBs a chance to develop?
There's plenty of throws to go around in the offseason and training camp, and then to your point very few throws available once the season gets rolling. Technical development can still happen in throwing sessions and classroom time is probably most valuable at QB. There are lots of QBs who developed largely on the bench, including of course Geno here.
It is a problem to fit in live fire reps, especially with how few padded practices there are lately. However, as padded practices continue to decrease then evaluations will need to be done without them anyway. Walkthroughs, how they perform on the whiteboard, and throwing sessions will need to be enough to evaluate a single young player, so I don't see why you couldn't do the same with multiple.
At the end of the day, until the next CBA and any potential changes to the rookie pool allocations, there is such a discrepancy between free market QB prices and rookie QB prices that drafting QBs seems like an obvious strategy that anybody without a cheap young QB should be leaning into.