This is too cool to be true, on a1k.org user Ratte have made himself a very professional looking automatic monitor switch so that Amiga can switch between native screenmode and RTG-modes just as if you where running a CyberVision64-3D with scandoubler or Picasso IV.
I guess this device is specifically made for the Indivision, but it should be easy to modify pinouts to work with other scandoublers (however why would you, Indivision is the best).
You can be sure I am already saving up euro for at least one of these cool switches 
Automatic monitor switch on a1k.org
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Tags: a1k.org, flickerfixer, Indivision, Ratte, scandoubler