I am guessing you have the same selectable association in both selectors?
Therefore when you set the association, the selected value is visible at both (which is correct behavior)
If you create an alternative association to the same object you can have users select this in your second reference selector.
See the picture for an example:
It sounds like you may want to model this association as many-to-many instead of many-to-one. If that is indeed the case, you can use a Reference Set Selector widget to allow multiple values to be set for the assocation.
Hi Wieke and Philips, thanks for your answers!
@Wieke, i tried it your way, but it didnt work out for me. I added some print screens, maybe you can see the problem.
@Philip, I think it is not possible to only use a many-to-many association when you use a reference selector.