The Phoenix FD RDC is capable of being a backup dispatch center for the Mesa FD RDC. I've heard them do it earlier this year.
However, typically, what you'll hear is mutual aid assignments, either from Tempe or Chandler into Mesa or Gilbert, or Mesa or Gilbert into Tempe or Chandler. This way, every responding unit is aware of every other responding unit.
And Mesa has licensed a handful of new VHF channels, so it does appear that at some point in the future, Mesa will have analog VHF channels for hazard zone incidents, as well as their digital trunked system for EMS incidents, just as Phoenix does.
No, I don't know what the timeline is for them to go live.