I've kinda managed to have my sims ride together by using the "Trot here" option on each mounted sim, usually with slightly delayed timing so they don't clip into each other. That way they do usually choose the same path.
I had my lone traveler Logan and his daughter Alta ride together to go swimming (it was a very short swim since I forgot it was winter ?)
I've discovered that watching my sims ride together is one of my favorite things about the pack, so it's sad that there is no real mechanic to see that happen. I still hope it may be added with a patch later on, because it would be sooooo good!
https://media.discordapp.net/attachments/1137792374806958171/1138168033400066200/189d0c473d684-screenshotUrl.jpg