What will likely matter more than hardware interrupt priority is how the software handles each port differently.

Normally, the SCC is configured to generate an interrupt per byte received. This would be an excessive amount of interrupts for localtalk at the speeds it operates at. So, the printer port is expected to be used for localtalk, and when it receives an interrupt indicating a localtalk packet, interrupts are disabled and it polls in the rest of the packet. This means interrupts can be disabled for an extended period of time, and could potentially miss data coming in from the modem port. So the localtalk driver also polls the modem port for data while polling the localtalk packet in, and buffers the modem data so it minimizes the chance of dropping data on the modem port while polling in the localtalk packet. The floppy driver also does this, since floppy accesses are performed with interrupts disabled.