Why does the dragged MIDI part Replace/Overwrite instead of Merge?

Hi, I’m trying to combine an overlap of MIDI between 2 parts. Whenever I drag MIDI part B onto the end of MIDI Part A, it deletes/overwrites that section of Part A. How do I make the data merge into one part? I looked over the Prefs and couldn’t find anything, and changing MIDI Record mode to Merge doesn’t do it either.