The most pragmatic way of setting up desynched bairs that I can think of off the top of my head would be via the ftilt guard desynch: wavedash while facing your opponent, turn around ftilt, and start the desynched bairs. Similarly, a pivot desynch that ends with Popo facing backwards might also be okay.
I can't seriously advise ever using desynched bairs like that, though, since it seems so fragile and uniformly impractical. Desynched bair shield pressure is not very threatening, particularly if you don't fastfall. My general stance on shield pressure with ICs is that if the opponent ever sits in their shield for more than a very brief period of time, just grab them; if not, figure out how they like to escape their shield and punish. Opportunities for weird, complex shield pressure generally do not happen with ICs, and even when they do, I do not believe they should be pursued.
Anyways, if you do want to do desynched bairs for one reason or another and you want to know your follow-up options, they vary a lot based on the opponent's character, percentage, DI, etc. If you hit with a Popo bair, you probably will very rarely be able to do anything substantial, if it all, with Nana to follow it up, but you still might be able to just wait for Popo to land and chase the opponent when synced again. If you hit with a Nana bair, you probably could combo off of it at with Popo at low percentages.
Still though, I can't recommend using bair this way since I see no situations in which this is better than some standard option. Of course, there's no harm in experimenting with it if you wish to.