Don't.
Assuming that the two controls are independent (i.e. one doesn't contain the other) then you should indicate the click to the control containing both, and it then tells the second control what to do.
Have a look at this:
Transferring information between two forms, Part 3: Child to Child[
^] - it describes the process for Forms, but Controls are exactly the same (indeed, a Form is an instance of a Control)
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