Multi-file movie has title like first file name

[OttoKerner]

Just rename it.
https://support.plex.tv/articles/201272763-edit-details/

Or check if the two files are able to be linked together, which is the better option.

[HowTo]: joining multi-part movies files with MKVtoolnix GUI

The purpose of this HowTo is to join videofiles, when a movie was ripped and split into two or more files. (This happens usually with very long movies [i.e. LOTR] or more often back in the day when we had Video CD or DIVX CDs) The ultimate goal is to have the movie back in one file, so there is no trouble in the various Plex clients which do not support multi-part files. The tool used is MKVtoolnixGUI it is freely available for all major PC operating systems. Therefore the container format …