> Probably because I'm a threading/multiprocessing neophyte :)

That's a very good reason :-)

Here's a version using two multiprocessing events. Note that I use
timeouts for wait() just to avoid being stuck if something goes wrong:
the test now runs in a dozen ms.

By the way, next time you need a duplex communication between two
processes, you can use socketpair(), which returns a pair of connected
sockets.