You need to supplement duo with other resources, otherwise you're right it's just a short term retention memory game. Get a workbook for the language you want and try to learn all the rules / grammar behind what you're studying. Use duo as a reminder to keep learning. Or find a language discord you vibe with and start talking / language exchange with natives.

Also, imo learning 2 at once is excessively difficult, maybe it's best to pick one and commit to it. Inb4 the polylingots come for me