I need a device (either an ISA board or a subsystem) which will
take two RGB video signals and combine them according to a template.
The template can be as simple as a rectangular window with signal
one being used for the interior and signal two for the exterior.
But I beleive fancier harware may also exist which I do not want
to exclude from my search.  I know this sort of hardware exists
for NTSC, etc. but I need it for RGB.

Please email and or post any leads....