Hello,

I think the spillover won't work in the way you expect.
Actually, i have not tested it myself since i only have one M-6R and one M-1000 (poor me ;)
But don't expect undocumented features... Moreover, Oberheim did change the spillover mode to 'unit mode' in the M-1000. This leads me to think they found that the Spillover mode was not satifying...

From the manual, i understand that :

Press six keys : device 1 plays the six notes
Press a seventh key : device 2 play the seventh note

There is no mention of polyphony management during note tails, only for key press events. So, I can only imagine that in your case :
Press six keys : device 1 plays the six notes
Release all keys, and during long release sound
Press one key : device 1 steels from one of the six still playing note to play the new one.

The Matrix-1000 could be a better choice in your case (btw the manual states you can stack up to 6 M-1000 !!), because it works as:
each new key pressed is played in turn by the next device in the stack.

In you case again:
Press six keys : device 1 plays notes 1 3 5 on voices (1-2-3), device 2 plays notes 2,4 & 6 on voices 1, 2 & 3
Release all keys, and during long release sound
Press one key : device 1 plays note 7 on voice 4.

But don't trust me blindly, ask if someone with two M-1000 can test it for real...