You are right, sorry for overlooking this a bit.
You can locally achieve your goal in Engrave mode. Select the 8th notes (ac) in your second voice, open the properties panel and enable voice column index. Put it at 1 and both voices will overlap there.
Hope it helps.
I don’t understand Dorico’s behaviour in this instance though. If it’s intended or a bug somehow. It works just fine when both voices simply contain a single note (as in my new attachment), but obviously not when both voices contain two notes. Shouldn’t it work the same way in both cases?
Dorico won’t allow two noteheads to overlap because it doesn’t look very clear with both stems extending to the outermost note in both directions on either side of the noteheads. It ends up looking like a big black blob, and it’s hard to tell at a glance which notes are actually to be played by which voice/player.