User 1 has modified file A and B and has a lock on A.
User 2 has modified file A and B and has a lock on B.
To check-in, your files cannot be locked by another user.
Figure out what files are checked out by each-other and unlock them.
You can unlock via:
Reverting the change to the file on 1 user’s machine.
Unlocking the file using the right click Revision Control → Unlock option (also available in the CONFIRM SUBMIT dialog). These files have a red lock icon in the list.
Note that you’ll likely end up with an asset conflict that you’ll need to resolve.