Can you default members in the Page of a form based on the selection of another member?

We have two dimensions in our application which are virtually identical, they have the same member names (but different prefixes).

Is it possible that if a user changes the selection of one, that the other one automatically changes? e.g if they pick XX_123456, then YY_123456 on the other dimension is automatically selected?

The valid intersections I do not think will be an option, because they need the ability to change the "YY_" members if they so choose, but by default they want it to the match the "XX_" members.

I assume groovy could achieve this, and if anyone has the code I'd be happy to implement it - but timeline constraints makes it hard to develop it from scratch when you aren't