I think it might be helpful to have a cross-selection mode when we are in editor-viewer split mode - to get text selected in the editor when you select it in the viewer.
The reason why i'm straggling with it is that sometimes volume of text in these modes could be very different because of formatting, especially when you have a huge table. (this one on the picture below not a best my example but the first I found). So when you wish to format text you have to find it in the editor or go to RTE for process it there. When you are in a split mode and wish to format text selected in the viewer program adds directive in the editor section on position where cursor was last time. (see ==marked text== on the screenshot)
Also, selection in the viewer mode can be used for ctrl+c only. I think, it will be good to have a choice to enable some formatting features in this mode, for instance highlighting, which I miss so much when working with text. Thankfully, we have RTE, but it is different, and having the way to do some annotation in a markdown viewer will be way better.