Double stops with different note length [SOLVED]

I'm creating a cello score with double stops. Here's an image of what I have:

The measure on the left has a single long note at the bottom and several shorter notes at the top. That's what I want and MuseScore made it like that automatically when I imported a midi file. However, I have no idea how to make that myself in Musescore. When I remove the two sixteenth notes with the red arrows and put an eight note there instead, then Musescore replaces the higher notes with one eight note as well. I want those higher notes to remain in place. I just want to merge those two lower sixteenths with the arrows into one eight note. How can I do that in Musescore?

(I know it doesn't matter much in this particular case, but I have other measures with lots of notes at the top and just a single long note at the bottom.)