After some brainstorming I've come with the following solution:
shape (has qualifiers 0, 1 and varA)
Basically I refer to two different instances of the same active trough different qualifiers and differenciate them with varA.
If qualifier 0 collides with qualifier 1
If qualifier 0 has varA = 0
If qualifier 1 has varA = 1
- then destroy qualifier 1
I didn't tested it so I'm not sure if it works, but I will try later.
I am still interested if there are some ways and if I can pick an instance of an active during an event.