Wrong Azad.
Thats exactly the point.
I DONT want to join the panels, otherwise i would simply use a larger panel to accomodate the window/door in a single panel.
I wanted exactly to cut multiple SEPARATED panels at the same time, using just one window/door. And to do that the only way i found (after a exhasuting research in autodesk forum and the rest of the internet) was this simple process that i explained before, to 'trick' revit to assume the cut in multiple panels is possible - By joining the panels, the void in the dorr/window can cut those multiple panels (because revit assume that is just one big panel, but it is not - its 2 panels that are 'joined'). And then, when you Unjoin those panels again, revit separate them again and remains the cut geometry, introducing the curtain wall grids that were deleted before (when you joined the panels).
This a very unique theme that so far there was no way to do it. I already talked to many architects coleagues that could not find a way to do this.
Today i needed to do this, because im working with insualted panels, and i need to deliver shop drawings and Quantity TO for the factory with the precise number of panels, and precise geometry of all the cuts in each panel.