merge two kernel .config 's into one union of the two

I have two quite different kernel .config 's and would like to make a single 
union of the two such that if any feature or device is enabled or suppported 
in either, then it is in the union, but also (of course) a valid .config 
that make oldconfig  will accept.

Does anyone have a tool that can do this?

Or failing a tool, can anyone suggest any method at all for achieving this?

John