There are two things you'd merge: changes to the whole presentation (i.e. new slides), and multiple changes to the same slide. In the first instance, we slot together the slides and alert the user. In the latter instance (if you sync a change and I sync a change to a single slide), they will both appear chronologically in the list. It is up to the user to merge these versions together.
Automatic merging works well with code because you can move locations of functions and -- to some extent -- chunks of code (although git usually will require some merge resolution); as we are merging something (a slide) which, unlike code, has to appear a certain way visually, this approach doesn't make much sense. Auto-merging together two slides with different content would not have a desirable result. I'd love to hear your ideas on what you think the best UX for this is.