Breastfeed one side at a time or both?

I was told to try and feed from both sides each time from the hospital coach. Well now he eats so inconsistently I am struggling with that. Sometimes he eats for 20 mins each side. Other times he feeds for 20 mins one side and is done and then my other side gets super hard or engorged until the next feeding. My supply also seems very different on both sides. Any advice? Do you feed from both sides or one breast for each feed?