Since you can launch only one session of this application, you will have to install Sandboxie to open the software virtualized. Basically, what you have to achieve is to have two sessions of this software installed to be able to copy from one file to another since it lacks a merge function.
Install Sandboxie, launch a session of 4UOnly, then launch another one normally and open the files. Create one big master file and copy from one side to the other.
Secondly, try using a password manager that offers strong encryption, portability, multi-platforms. There's one that everyone is using and it's called LastPass.