I think when I’ve used the FlySky FS-IA6B receivers, each receiver was separately bound per model in the TX. Turning on the transmitter with a second receiver wouldn’t connect until you changed over to the other model in the TX settings.
Just to be sure, did you bind to the first receiver, then remove the bind plug, then turn off the transmitter, and then switch to a different model? They would all need to be bound one at a time, and each would need its own model in the TX. Then I think you could probably switch by changing the model in the TX. I could try if it wasn’t going well for you.