If you didn't have the link-to-entry field in the primary library, then you can use libByName() to get the
library object, but then you'd have to use one of the find methods or else do you own find by looping through the secondary library's entries to match to find the proper entry.
If you have the link-to-entry field in the primary library, then you can access the proper entry with...
entry().field(yourLinkField)[0].field(yourDesiredField);
As for whether to have the link, I consider a link-to-entry field to be appropriate when a normal, regular relationship (relationship == link in Memento) exists between the libraries. If not, then it's like reaching into a neighbor's things to fetch something you want. Not appropriate.
If anyone can tell me on what basis you'd want to take info from a library with which yours has no relationship, then I'll consider that and update the thread with my finding. It seems wrong to me.