J’ai dormi sur les deux oreilles.
zhay dore-MEE sewr lay DUH zoh-RAYuh.
I slept like a log.
Actually, there is another French expression that translates more like the English: J’ai dormi comme une souche (zhay dore-MEE kum ewn SOOSH, I slept like a stump). But I like today’s expression better: it’s more fun! Literally, you are saying, I slept on both ears.
I don’t suppose anyone knows whether this expression implies sleeping on one side and then the other, or sleeping on both sides at once, which of course is impossible. Anyway, if you’re sleeping that soundly, chances are you don’t know and don’t care which ear you are sleeping on!
By the way, there’s no word in French for both. You have to say les deux, the (or my) two (ears, hands, eyes, etc.).