As set up, NLM has the citation author lists set as if 3 or more, list 2 in both first and subsequent settings. (see image). you would need to change the two to a one in both spots, save as a new style and use that style in your word ribbon. You also need to add the et al. text to each (as shown in image and attached modified style).
I made the changes (and also changed the bibliography number to be in square brackets rather than parentheses, since that was what you showed and it was different in my NLM.
NLM (1authoretal).ens (43.2 KB)