1Click 'Open database' button on the left panel:
2Select your file and click 3'Open':
Wait a bit for a progress and then repeat the same for the right panel 4:
Wait a bit more for a progress again and here it is!
If your DB files are located in the same folder then this can be done even easier. 'Open database(s)' toolbar button allows to select two files and they are loaded in parallel.