Thanks to John D. Rutoskey for the help with O-roll instruments.  Okay,
so we have established that the piano should really turn off if either
solo instrument turns on.  The way I will do this is with a metal rod
behind the jacks of the piano, disengaging them when the piano should
be off.  This is an idea from Craig Brougher's book.

Now, I am still not quite sure if one solo instrument should override
the other when they both come on or if the rolls are arranged in such
a way that this will never happen?

Here is another question about O-roll spool frames: Must the roll
flanges (not takeup spool) be on a solid axle or can they be fixed to
two separate axles running in bearings in the side plates like a normal
player piano spool frame except with the flanges being part of the
spoolframe and not the roll?

From what I could establish so far, the flanges seem to be mounted to a
solid axle which can be removed as a whole.  My guess is that one would
need a screwdriver or wing-nut to remove the one flange in order to
change the roll.

Could someone please give me some hints as to what is the best method
and why.

Regards,

Bernt Damm