Steaming while boiling food

Can you use the steam from boiling one thing to cook something else? E.g. Boiling potatoes and steam broccoli in the same pot at the same time.

Yes, if you have the right pot, namely one that has a steamer insert that is well above the bottom of the pot, or even stacks.

In fact, in several cuisines this is the standard way of getting several ingredients ready at once. For example, it's common to steam couscous in the steam from the Moroccan stew cooking below, using a stacking pot called a couscousiere.

Just make sure that the food in the steamer will take less time to steam than the food below takes to boil.