Dictionary Merge (of the 'File Center' tab) can merge two textual lists containing one entry per line, that are sorted alphabetically and have optionally a count as the first line. Such files are for example used by the DSpellCheck Hunspell. DSpellCheck Hunspell is a generic word checker plugin that can be used by several programs, one of them is Notepad++, and also OpenOffice and LibreOffice support it.
The idea is that you put the DSpellCheck's files (the content of the 'Hunspell' folder, something like *.dic, *.aff and *.usr) on a cloud drive and set up the DSpellCheck plugin to work on them there. *.dic are the downloaded predefined dictionaries while *.usr are your personal additions to the dictionaries. Without reconfiguration you have multiple Hunspell folders, one on each computer. With this function you can merge such dictionaries effortlessly and then put the result on a cloud drive. You only have to merge all the *.usr files, they are the ones that contain your added words. So as input for DSpellCheck you would enter the expression "C:\YourHunspellFolder\*.usr" in 'Source File Pattern'. This is already preconfigured by pushing the Default button just right of the box.
But this function is not limited to Hunspell dictionaries. It's generic and can be used to merge any two textual lists. For example you could have two lists of names, or two lists of movie titles or other lists exported from applications. The lists can optionally have a count of the following lines as their first line, but not required. By the way, for the management of a list of your seen movies there exists a more specialized app available from us that does this more sophisticatedly, it's called Atlas Subtitler.