This is subtle, but another aid that Dorico provides is built right into the caret.
In the following gif I first press enter, and then tap V a few times to cycle between the upstem 1 and downstem 1 voices that are already present in this score. This means that when I activate note entry mode, I can choose which voice I want to add notes into. In this particular example, if I stayed with the upstem option, I’d overwrite the top line, and if I leave it on the downstem icon I’d over write the middle c’s.
I then tap SHIFT+V to cycle between adding a new upstem 2 and downstem 2 since they are the next voices that can be added (you can tell you are adding a voice by the + icon). If I add a second upstem, I would obviously be adding notes that would coexist with what is there.
This information is always present when you enter note entry or attempt to add a new voice with shift V. Learning to recognize this change in the caret will help you avoid further confusion too.
