I created a simple demo to confirm my problem:
Now if preview is used (or form populated), we can choose a filter and both dropdowns "ItemA" and "ItemB" always contains the same value. The requests sent contains only a single query to the Items-List.
I used this abstract sample as explanation that this situation can happen.
In our concrete form the user selects two persons (not multiple) which are defined in the "Items-List". Each person contain many data fields and the persons are filtered by something like "person is X" and "person is Y". The user has to select one of "X" and one of "Y" in the form.
Actual workaround is to duplicate the complete person-list ...