I belive you can move the mous over on the secound wow instance, and then click the button for the mous over macro; wow should be handling the mouse position without the window being in focus, so no need to click either.
In other words; when you want to heal your tank, move your mous from the tanks window to the priest, click the button for heal (mousover macro) and it should go methinks. Although, I can't test that atm. And I don't realy like those kinds of macros anyways :s