Since .po files are full text, you could first compare the files to see if the differences between the files occur at a specific place (is the addition only located at the end?), are the same keys used for different values?, … to ensure that no data is being lost.
To that purpose you could use:
– CLI tools such as diff or a program like Meld (which is based on diff) to get a visual aid when comparing the files.
– an IDE which provides file comparison possibilities (For example Atom with the add-on compare-files)
You could try to merge the files together automatically by using the following CLI tool msgmerge.