How do you confine them without over heating them? I'm assuming that we are talking about blossom time when it can get warm. A time when the bees really want to forage and therefore will, probably really build up heat.
So you've confined them for two days. Now what makes them go to the crop that you want them to pollinate? Unless that is the predominant thing in bloom for miles aren't you going to get only partial pollination? Do you really expect to get more pollination on the other crop just because you confined them? It seems to me that the renegade foragers who aren't already working the first crop are going to get the colony to work on the next most appealling source of pollen. The next vegetable flower available. I could be wrong though. You tell me.
Everything works unless something is broke.
The answers are the end. The questions are the journey. Journey on.