I haven't been following this thread lately, but somehow the replies have gotten you
completely confused. In fact, your original ckts in posts #8 and #11 are ALL correct,
and since then, they are all wrong.
However, you possibly did not have a ground wire connected between the 2 processor
boards originally. So, forget everything else, and go back to your original ckts, and
where it says Gnd, just make sure that point goes to the grounds on both boards.