Again, I'm compelled to point out the edge-case of British Waterways advice to cyclists on towpaths, which is to always pass pedestrians on the water side. It makes good sense for cyclist/pedestrian interactions, but I've no idea how commonly known it is, and of course it doesn't help for bike/bike. I suspect that the ones who are canal boat users (and therefore more likely to be exposed to British Waterways literature) are more likely to be aware of it than 'normal' cyclists, and the ratio is going to vary a lot depending on what bit of path you consider.
In practice, there's either lots of room - in which case defaulting to the left usually works - or riders make an arbitrary decision about who deviates from the singletrack based on relative speed, politeness, bar width, load, off-road capability of the bike and so on. Most of the time this works surprisingly well. More so than on off-road paths where there isn't a straight choice between snagging on a hedge/fence or falling in a canal.