Is it possible for antennas A and B to track ODY together (creating an array), while at the same time antennas A and C track MRO together (creating another array)?
If you have three apertures pointed at Mars, then you would array all three for both orbiters. There would be no point in throwing away the signal of one of the orbiters on any of the antennas. You can array four antennas if you like.
The DSN catalog does mention that combining MSPA and arraying is good bit more work for them, so it has to be negotiated with them when planning the mission.
By the way "track" isn't the right word here, since that implies two-way communication for Doppler or range tracking. MSPA supports multiple spacecraft on the receive side, but only one spacecraft on the transmit side, per antenna. You are receiving data from multiple spacecraft simultaneously, not tracking them simultaneously. Again, per antenna.