You could, of course, us the filter in the "Edit Recipient List" and do separate merges that way. Of course, if there are 60 that would be a right pain.
If you're doing this repeatedly, I'm sure VBA could help, though I haven't used VBA for this particular thing.
There are some add-on tools for merge - Doug Robbins' for example - I don't know whether this will help specifically with this.
Also in this forum, there is an item towards the bottom on this Tips and Tricks page which looks like it might work by, after merging all your items into one document, helping to split it into separate documents afterwards?
https://www.msofficeforums.com/mail-...%20the%20merge