How about the nozzle assembly includes a paddle at the mixing point, a shaft that extends back, which sockets into a motor? The shaft extends back through a gasket (there's minimal pressure, ideally) and the paddle is rotated as the epoxy components are dispensed from their respective tanks, no matter how it's done.
Expoxy A
Motor and gasket ----]------{} =------ >>> mixed epoxy
/
Epoxy B
The nozzle and paddle simply are removed and tossed.