Short answer
Both are generally safe for dogs when served plain and cooked. Couscous is a simple pasta, while quinoa is a seed that contains saponins, which can cause mild stomach upset in some dogs if not rinsed well. Offer either in small amounts as an occasional treat, watch for any digestive upset, and call your own vet if anything seems off.
Couscous — Food safety facts
Safe to share · Dogs · full breakdown on the linked food page
Plain cooked couscous is safe. Watch for added stock or seasoning.
The full verdict, portion guidance and serving notes are collected on the Couscous food facts page.