Offering both breasts?

Up until now I've only been offering one breast at a time but my 5-week LO slept for a longer stretch (4 hrs opposed to 2 nothing crazy) and my other side felt engorged, so I offered it after he had already emptied one side and he drank for another solid 10 minutes. Should I be offering both to make sure he gets full?! I can't believe how much he just ate-- I was told not to let him BF past twenty minutes to avoid him pacifying and burning calories, but offering both breasts definitely went over that time limit. What's a mom to do?