There are few ways to sync the reading progress of sideloaded books across Kobo devices. One is to use the Kobo Utilities plugin. Yes, you can get the progress from one device and send it to the other, but IMO it's faster and easier to just remember a word or a phrase on the page where you left off, unless you read several books at once.
I use my Sage and Libra 2 this way - the same books are on both (I don't use the Reading List plugin, just a tag to mark which books are on my Kobos). As I don't connect my readers to my PC very often, I just try to remember where I left off, mostly successfully.
Another way to sync reading progress (via wifi) is supposed to be Koblime:
https://kobli.me/
I haven't used it myself, although I plan to try it someday, so I have no idea how well it works.