I was the idea to:
1- look for some signal in the MOBO of the AMIGA.
2- Or design a circuit that detected the presence of video in exit RGB of the AMIGA thanks to tooltype of CGX KEEPAMIGAVIDEO i.e
who can pilot the switch.
But i found this post:
"Sometime ago there was a discussion about how to manage the native Amiga
screens and the graphic card screen, both active, on one monitor.
Someone asked for a free memory location common to all Amigas to set via a
commodity that checks which screen is frontmost.
I proposed on the Picasso96 ML to use the Sel DF3 signal, by creating the
proper function inside the RTG software, to control an external switcher.
They answered me (Tobias, a P96 developer) that both CyberGFX and P96 have
such a function but realized in a more clever way: a sync signal (V or H) is
set to a costant voltage level when an Amiga native screen becomes frontmost.
But this system is depending on the chipset used by the board: normally all
those support DPMS are able to set the sync signals this way.
There are some commercial switchers available that work this way: the Javosoft
one, the Eyetech one and one for the Retina card.
Strangely this feature is not documented on the RTG manuals."
I want to investigate (but i not get time

)if this is certain in my system.
And try to design the circuit that perform the logic levels to control any switch electronic or even add this to any already exists schema of switch contributing a compact solution to AMIGAS with RTG Monitor problem.
Any ideas are welcome.
Sorry for my bad english ;-)