In a multi user environment, the database should be split with a shared backend file. However EACH user needs their own copy of the front end database.
If you have two or more users opening the same database at the same time you have a 'crash' waiting to happen. Just like if two people tried to drive the same car simultaneously.
In your case, you've just been locked out. However at some point corruption WILL occur.
So when I split the database I should not have an issue opening it while another user has it open, correct?
As long as each user has their own copy of the front end on their own machine, it should work fine in a multiuser environment.
You need to treat this as a priority and until then make sure only one person is using the database.
I concur completely with Ridders, it is a carcrash waiting to happen. Just as a suggestion as to why it works one way and not the other is that the other user as gone into design view for a form, report, table or module which automatically put the app into exclusive use - if you were already in the db, the other user would not be able to go into design view.The other user is able to open it just fine if I am already in it, but not the other way around
You are on very shaky ground with your current setup