Multiple simultaneous designers works great. The only catch is that they can’t both work on the same window. You can try this out by opening multiple instances of the designer on one machine. It works the same way with separate computers on the network.
Offline editing is much trickier with multiple people since you’re effectively working on a backup. If only one person is working on the project and making significant changes they would want to do an entire gateway backup and restore. Multiple designers could do small tasks like create/modify windows, then do a window import. I suspect that you would face similar issues as multiple people working on any programming project, document, etc without connectivity to each other. The hard part is in knowing what is current and managing the merge.
Concurrent development works most smoothly when everyone is working on the same place. This could be a development or staging machine.