"have the master uC command the slave"

Yes, that's one approach.

Another would be to have the "slaves" (peripherals) run more-or-less autonomously, and just report status back to the "master" - possibly unsolicited.

Note that a multiprocessor system does not have to be a master-slave arrangement - but as you're talking about "peripherals" I guess that's what you intended in this case?

The 8051 has a special serial port mode specifically designed for communications between a master & multiple slaves.