Other than the fact that you have to share logins, there isnt really another downside to just using one account. If you want to use 2 accounts, there are a couple of issues.
1. If you use 2 accounts, you can add a second driver to each account easily now, without having to contact tesla. Have the user create a tesla account, then access your tesla account and add them as a second driver.... with that being said
2. The "second driver" does NOT get the "roadside assistance" icon in their app, which I find is a big deal. My wife is my "second driver" and if she had an issue, i absolutely would want her to be able to contact roadside assistance, and that is basically app based at this point (although you can call as well its just as fast to use the app)
Because of point 2 above, I went back to just sharing my login info with my wife. I just like to separate login information because its bad practice to share logins in general, not because I wanted something specific. I believe there is a way to contact tesla as the owner of the vehicle and ask them to add a second person, but I have not done that. I suspect if they do it that way, maybe the second person will get roadside assistance in the app, but like I said I havent tried.