Merging data from two MacBooks with one Apple Account

Probably the easiest would be to use iCloud in fully activated mode to sync items between the two. If she is indeed using a single Apple Account then that would have happened unless she had told each Mac to not sync contents with iCloud. You can do a fair amount of controlling as to which services are used on which devices. She may decide that she need not want to sync all data to all devices. I don't need everything on my iPhone, for example so I have things such as documents syncng disabled on it.

Change which apps sync and store data with iCloud - Apple Support

The disadvantage is if she ups her iCloud usage beyond 5GB (a paltry amount these days) she will have to subscribe to a higher storage level. Read this support article for instructions on how to upgrade to iCloud+ --> Buy iCloud+ or upgrade your current storage plan - Apple Support

- The article includes instructions for various hardware and managing your plan.

All you have to do is turn on the iCloud feature you want to use on all devices you want syncing to that account. Files won't appear instantly but they should eventually sync. For example, I have my computer and my iPhone set to sync Contacts. As long as both can connect to the Internet at some stage, if I add a contact on my computer, it eventually shows up on my iPhone. If I did the same for my photos, however, I would easily exceed my 5GB free and would be asked if I wanted to subscribe to iCloud storage. I do have my photos sync from my iPhone to iCloud, but not the 40 GB or so on my computer. If i want my iPhone photos on my computer I can download them directly from iCloud to my computer using a web transfer and not risk any photos going from my computer to iCloud and filling it up.

It's that simple (you asked for "easiest" ;-) )

I only use Apple's smaller things for iCloud syncing, such as Contacts. For those I do not see any device specific versions. As for other things, I only know what is says in Apple's documentation above, and at Change iCloud settings on Mac - Apple Support