Maybe what Karl meant (or, what he could do) is something like that -- eg,
pre-position ATC to empty pocket left of next tool y; remove current tool
x from spindle; park x in empty pocket; shift ATC left; pick y; install y
in spindle and start mill ops; shift ATC right; pick x; shift ATC to home
pocket of x; park x; repeat ad lib.

This route would work and probably be safer. I really haven't serously
thought it all out. The other concept is just always put the tool in
the closest empty spot to the next tool you need. My control has look
ahead so this isn't a problem. But you would always be changing your
tool table as to what tool is where. Screw up, even once, and break
some serious ****.

Karl