The newest update would win.
Though that would have to be down to at least the second, maybe smaller.
I'm not positive on this, I've used Joplin with Syncthing with no issues.
I think by default Resilio has a tighter sync timing than Syncthing (though ST is adjustable) - how sensitive it is in time would be the concern.
It will consider the newest change to the file to be the one to use.
It does seem to create a new file when a change is made. Not sure when it cleans up the old one.