If you are planning on using the pollen from the one tent to make seeds in the other, on the same grow, I would flip the reversed plants as soon as possible and a few weeks before. The plants don't have to huge to make pollen. (as BoatGuy says yo udon't need alot) It varies but it takes at least 4 or 5 weeks after flip to get much pollen out of the reversed plants and sometimes longer. Flipping the pollen plants first makes sure you have a good amount of pollen before the plants you are making the seeds on have flowers ready to be pollenated.
You want to give time for the seeds to mature on the plants after pollenating.