Manage Authors lists Authors as individuals not as joint authors.
Ah, I think you mean Edit Authors.
Not sure I even knew it existed, don't use MDE. For me drag-n-drop is hard to use, especially in lists.
Opening modal MDE, then right clicking to get into the Edit author modal, then drag-n-drop in a list, then doing whatever to back out of the modals seems like a lot of hard work… compared to using F2, then C, V, →, ← keys with Shift and Ctrl modifiers.
BR