I don’t think delimiters other than comma can be used for author entry. Delimiters for Author List are greyed out and non-editable. These delimiters, like tab, dash, and semi-colon, are delimiters for the custom term lists such that multiple terms are entered in a single line.
Endnote author format (for the data entry) has been very specific and fixed: (Surname), Initials of given name with space between. So, it should look like Smith, A. G.
Anything before the first comma is regarded as surname, and anything after the first comma is regarded as given name.
A G Smith is also acceptable entry format, but most of the import filters parse authors with Smith, A. G. format. Also, each author needs to be on separate line. If you have multiple authors in one line, it will not interpret correctly.
I’m not sure how the initial post ended up Smith & J (two authors).