The important question is why would you want to do that? Do you need more I/O pins? ... There are other ways to get those. Do you need more space for a bigger program? ... There are other Stamp models with multiple "slots" each of which stores a program up to 2K and these programs can transfer from one to another ... sort of like overlays ... there's no CALL / RETURN, just a JUMP from one slot to another. Do you need two boards so you can do two things at a time? ... Think about using a Propeller instead which can do 8 things at a time.
Comments
Each Board need a own program, but they can communicate with each other thru for example serial coms.
Enjoy!
Mike