Last year I had two oneworld tickets in use at once, and another return ticket.

CBR-SYD-NRT/HND-ITM-NRT-SIN-HKG-JFK-BWI-JFK-DFW-ORD-MCO-DFW-SEA-JFK-CDG-HEL-LHR-DXB-LHR-HKG-NRT-SIN-PER-MEL

Red: DAS13 (issued by QF[AU])
Blue: DONE3 (issued by QF[JP])
Green: return (issued by AA[AU])
no one had any issues with it (other then the confused Japanese immigration agent).