Dave- The separate grounds allow the Amiga to interface with much less
interference by running Coax to each R,G, and B inputs on an RGB monitor, but
I believe you'd be able to combine them without any problem. And although I
don't have a pinout guide handy, I think they have another sync that allows
something like separate horizontal and vertical sync. Maybe someone could
post the pinout of the 23 pin connector. But I think you're biggest problem
would be the monitor. I've heard several places that Atari is using a 70Hz
refresh rate on its monitors, instead of the usual 60Hz. If this is true,
then the monitor won't sync properly with the Amiga output, which is standard
NTSC. The 70Hz rate may only apply to their monochrome monitor, in which
case you're OK, or its (remotely) possible that the monitor is adjustable to
60Hz, though since its not sold separately, but only with the ST, there would
be no advantage for Atari to make it adjustable.