How to merge glyph sets?

I have two glyphs files with glyph sets that don’t overlap, what’s the best way to merge them? I’ve tried copy-pasting but the glyphs show up empty…

Edit: problem identified: I had one master in one font and 3 in the other, I was trying to copy the 1 master font into the 3 master one. I added some empty masters and it worked.

(my apologies for my recent posts that include me solving my own problems…)