Thank you for your reply, Michael.
I actually tried this aswell but I didn't manage to work it out. The problem was how to syntax that the trigger should select the already selected Values in Field 2.
Luckily, I found a working solution using alternate states:
Copy selection in <Standart> state to an alternate state, if vselection=1 is selected.
Make all necessary selection with "Select Field" Trigger.
If vselection=1, copy back alternate state 1 to <Standart>.