in terms of seeing both masters in preview mode - my guess is that Glyphs Mini works different from Glyphs.
Oh well, I’m late as always 
Anyway, perhaps this will help in the future 
@elena 
thanks !
@mekkablue no, I don’t want to select path through layers, I just wanted to se the preview of the 2 masters (main + shadow) it is not a big deal, I give up 
BTW, I exported the font as usual, and I thought it would create 2 otf files (one for each master) but there is only one file (the main style) in the Adobe Fonts folder… Is there anything to do before exporting ?
You can turn on the preview at the bottom. or you can try one of the new Color layers. They are not yet covered by the tutorials.
Did you add a color to the first master?