there are cable doublers, then the 1 lever will operate both brakes off one lever.
Back in the day tandems had both rim brakes on a 2 cable brake lever,
and the drum drag brake on the rear hub, operated on the other lever..
so it's not un heard of..

A bit of a set up challenge, of course...,
its hard to be abnormal..

interruptor levers spread the Housing, that they interrupt.
the cable passes straight thru
to the brake at the end of the cable..