Since I wanted the same for my A1200D that I'm currently building I tried KVM switches but the quality sucked badly for my taste. It was more blurry than the original which I didn't liked at all.
So I tried the BMON/SMON switch by AmigaKit that totally rocks. It's expensive though but works just fine with the internal connector of BVision and I hope with Indivision 1200 also.
I yesterday tried it fitting it in the original A1200 backplate (where the SCSI from Blizzard accelerators goes) and works like a charm!
This is of course an Manual switching between RTG/AGA.
There is also a cool way to switch by putting an EZKey XS adapter (external PS2 keyboard adapter) that has the ability to use the blank keys in A1200 to give a signal. This signal can be inputed to the BMON/SMON switch and voila! Change of RTG/AGA with the click of a blank key!
Sadly the adapter that AmigaKit sells (and that I have bought to test it) is an SE and not XS which means cheaper version that is not compatible with that feature.
(You can see the EZkey SE adapter in the first photo of mine next to the keyboard ribbon connector)
For automatic switching I haven't heard for any way of doing it in an A1200.
