Trouble with Shared Noteheads

I've been having trouble with using shared noteheads. In this piece I'm trying to copy out in Musecore, there's some sextuplets made up of semi-quavers with the top notes - D, C, B in this example - noted down as quavers. I've used two different voices to notate it down, but I can't seem to get those notes to share heads with the semi-quavers.

I've looked at the handbook and tried using the invisible option, but that doesn't seem to work, and I couldn't find this option --> in the Note section of the Properties toolbar change "Head type (visual only)" to that of the higher value note.

If anyone knows any way to solve this, please let me know because it's been driving me mad! Thanks :)