If I am understanding your question correctly, you have two files hosted, FileA and FileB and FileB is related in some way to FileA. When Client1 logs into FileB using their credentials it is also prompting them to open FileA since it is related but since Client1 does not have credentials in FileA they get stuck and don't know what to do?
If this is the case, Client1 would need access to FileA as well if it is related to FileB. And typically, to make it easier on Client1 (or any client), you would have the same user credentials in both files so that Client1 (or any client) would only have to enter their credentials in one time.
If this is not what you are describing, we will need a bit more information to figure out what is going on.