hi chris,
do these two users open the excel document in their local onedrive for business sync folder and then the second user got the error message like below?

if yes, i’d like to explain that it is a normal behavior. after the first user edit the document and click save, then the change will be updated to the server. once it is finished, the second user should open the document again, then he will get the following
message:

click reopen document, and then the document will not be locked and he will be able to edit it again.
by the way, i would suggest you coauthor with each other online other than in local sync folder, which is the best practice and will not bring the error you described. for your reference:
document collaboration and co-authoring and real-time co-authoring in the excel web app: why and how we did it.
thanks,
sky