concatenate two values, each causing output when changed

Hi.
What I want to have is a two numbers message (2 3) that changes when one of the two numbers is changed, whatever which one. I want to look for a value in a coll with a (nth $1 $2) message.
I tried [zl join], but one of the two inputs doesn't generate a bang, [sprintf] neither. I tried to force bangs at some nodes without success.
I can't figure out how to make this type of message (nth $1 $2): the first number changes whatever number is changed and the second remains 0.

Here are my trials:

I suppose the answer is not far, but I might need some help..
Thank you!