Merge Photos (including edits and folders)?

You have a couple of options, both (may) have some drawbacks.

Firstly, you could export the modified versions from photos, album/folder by album/folder, but this may take some work. Secondly you could use iCloud photo library to help you merge libraries, how you do this may depend on whether you already use it or not, unfortunately I'm conscious that there have been reports that not all types of edits are transferred by iCloud photo and I'm not sure which are and which aren't.

unfortunately I'm conscious that there have been reports that not all types of edits are transferred by iCloud photo and I'm not sure which are and which aren't.

Do you have a link to such reports, Winston Churchill? It would be good to know more, in case that there are questions about missing edits. I have not yet seen any questions about this problem.

I am syncing several macs with iCloud Photo Library, and all edits are transferred between the Macs, even the edits with Photo Editing extensions, as long as I am having the same Photos version running on all Macs. Just the Faces albums do not transfer, which is annoying, but not yet supported.

But the syncing between the Macs supports more metadata than syncing to iOS devices. It supports albums, and smart albums, titles and captions as described here: Use Photos and iCloud Photo Library on multiple Mac computers - Apple Support

https://support.apple.com/en-us/HT204486

BTW, I posted a user tip on merging libraries in Photos for Mac: Notes on Merging Photos Libraries

Other than iCLoud Photo Library there is no good option, if we want a lossless merge.

If we do not mind to lose the ability to revert to the original, then PowerPhotos is a really good tool for moving photos between libraries, but it cannot move the originals and the edited versions as a pair. But it can move albums and metadata with the photos between libraries.

Do you have a link to such reports, Winston Churchill?

No, i've seen it mentioned a couple of times but if you've been through all the edits and not had any problems I guess thats a pretty good indication there may have been other factors involve in the cases I've seen mentioned.

Thank you, Winston Churchill. I hope there is not really an issue with syncing edits between Macs; maybe there have been problems with a delayed sync or conflicts, when the same photos have been modified simultaniously on two machines while offline.

Does it matter in which order I do it? Which MB should I do first?

You can't transfer faces or projects over the library in the cloud, so if you aren't worried about either it doesn't really matter except that if you enabled it in your main library first, all those photos would need to go to the lesser library so that the lesser library could put it's photos up there too, so you may wish to do your lesser library first.