A single certificate is required to intermix crews, and I would think that would apply to mechanics also. Contracts and seniority can be worked around, though not easily, so all three are normally needed to intermix. For other employee groups I would think training would be all that's needed to intermix operations, especially airport customer service and the like.
If any group wants to stay separate as long as possible, arbitration and the pace of contract talks are the only real means of doing that. To stay separate permanently requires the company's willingness to do that.
Note that for unionized groups, arbitration only comes in with either the union's merger policy (if the same union represents both sides) or the new law (if different unions represent the two sides). If one union represents both sides but doesn't use arbitration as a last resort in reaching a single seniority list, there will be no arbitration but those unions generally specify how the two groups are to be combined (AFA stipulates DOH, for example).
Jim