Hi and welcome to the forum! Tell us more about the parrotlets and your particular bird setup and experience. Every bird is an individual, so the more information that you share the more likely someone can give you helpful advice.
Hi there and welcome! Generally, parrotlets will bond quite strongly with their chosen one (and I do mean “one” - they are known to choose just one person or Parrotlet to bond with) but it is possible for them to bond with someone else if they are separated. It would be challenging, though, it the other bird is still around. Separating them (like having them in separate cages) would unlikely achieve your objective and could indeed be very stressful for them.
But those are just a general comments. As Dana said, it would be helpful to know more about them and the situation. How old are they, how long have you had them, how long have they been together, what is their living arrangement (same cage, cages side by side)? The more detail you can give us, the better.