JCBA is a process to merge the two contracts. There are three parties, the company and both MEC's.

Should start with an agreed transition agreement, basically a roadmap and timeline. This may include some agreement from the company to not do certain things, like shift block hours to the cheaper pilot group.

JCBA is not a normal "section 6" contract negotiation, it may only address parts of the contract that differ. Also the unions can just agree to keep certain sections of whichever is better contract in that regard, vice trying to re-invent the wheel. Especially with two similar NB operators.

Each MEC approves the TA separately, and then the pilots of both groups vote as one block to ratify. This is important because the larger pilot group can out-vote the smaller group, but only if the smaller group's MEC approved the TA to begin with... better trust your NC/MEC.

SLI is between the two MEC's, company is not involved. They can in theory reach a negotiated SLI, but in practice that has never happened, so it will ultimately go to arbitration. With two ALPA carriers, that will be based on ALPA policy, which is more restrictive than the over-arching federal protections.

Hard reality: Furloughs are probably getting stapled. Everybody else will mostly be slotted by longevity... that's not quite the same as DOH, since your longevity for SLI purposes can be reduced if you had furlough time, or even LOA in some cases. If your longevity is reduced, that will only apply for the SLI, it doesn't reduce your payscale, vacay, etc longevity.