If I'm "doing it wrong" and there's a way to materially speed up my dev flow, please enlighten me. I can't wait. :)
Generally, we avoid any merge conflicts with XIB files. We have a small team so basically anyone who is dealing with it has an 'exclusive' lock until they are done.
That includes avoiding having a feature branch that makes changes to XIB files that might change in the main branch. Merging XIB file changes isn't for the faint hearted - and the likelihood of having a working XIB file at the end of a merge is not 100%.
I wouldn't want two devs working on the UI at the same time. It's kind of an unspoken rule because just merging anything UI related is a big pain.
Like from the web world, one dev would do the UI html and js, while the other would do the server logic and API.
I try to use the MVC model in my projects, where the XIBs and their accompanying .m files just have basic getters/setters and the xibs are rarely changed.