What I think you're asking is:
You reference a control in a form without issue, such as a button click firing-
ThisEvent.Object (gives a reference to the button)
or if it was the form's onload, the same thing would give a reference to the form itself.
So you want to reference a different control than the one that is getting its event fired, which could be done like this:
ThisEvent.Object.Parent.Controls("ControlName")
Adding the .Parent gives you a reference to the page (one heirarchy level up), or form (two heirarchies up) if you were to do:
ThisEvent.Object.Parent.Parent
Then you could get a reference to the form, and once you can reference the form, access the other controls / pages the usual ways.
Hope this helps,
Eric