I'm not familiar with the plotregion.m function, but the first thing I would do is check to see if returns a graphics object handle. If it does then you can collect all those handles and set their Parent property as needed.
If the function doesn't return a handle, then the best solution will depend a lot on how plotregion is written. First I would just try this:
figure(1)
plotregion([-1 0 0;-1 -24 0],[0; 72],[-50;-50;50],[50;50;50])
plotregion([-1 0 0;-1 -24 0],[0; 72],[-50;-50;49],[50;50;49])
If plotregion uses only low-level graphics commands (like patch and line), then that should work. It should keep adding objects to the same axes.
If plotregion uses high-level graphics commands that respect the hold state, then you could try this:
figure(1)
plotregion([-1 0 0;-1 -24 0],[0; 72],[-50;-50;50],[50;50;50])
hold on
plotregion([-1 0 0;-1 -24 0],[0; 72],[-50;-50;49],[50;50;49])
If all else fails, you could probably try this:
figure(1)
plotregion([-1 0 0;-1 -24 0],[0; 72],[-50;-50;50],[50;50;50])
a1 = gca;
figure(2)
plotregion([-1 0 0;-1 -24 0],[0; 72],[-50;-50;49],[50;50;49])
a2 = gca;
objectsThatWereJustCreated = get(a2, 'Children');
set( objectsThatWereJustCreated, 'Parent', a1 ); % move them to the first axes
