I've used a line from master to slaves' reset. Master releases slave, waits for comm line state as ready signal, then sends & waits for reply (ack). Ensures slave sync at cost of free running independent tasks. My use: download new op data to slave, while master manages RF data link.
Can also use line to hit interrupt that sets a flag, then wait for max latency of flag (or signal/state) to start comm routine. I haven't done this one.
Multiple micros seems difficult, but can solve realtime conflicts when multiple interrupts aren't good enough. Good Hunting... <<<)))