afni.nimh.nih.gov

When you combine the 2 hemisphere volumes, do not
simply add them, but do something like -expr 'step(a+b)'.
That makes the result a union mask say, rather than a
sum.

After that you can add them (maybe with 3dmask_tool
-count, or 3dMean -count), and have a limit of 15.

Does that seem like what you want?

- rick