HACK - I attached my MAJGULL blackout curtains to my SANELA curtains using leftover RIKTIG wire curtain hooks. Way easier than everything that I was thinking of doing. It looks and works
MAJGULL Room darkening curtains, 1 pair, light gray, 145x250 cm (57x98") - IKEA CA | Cortinas blancas, Cortinas metalicas, Cortinas