Hot water only when multiple sources turned on

When one hot water source is turned on, the hot water is only lukewarm. But as soon as another source is simultaneously running, the water runs hot. For example, the kitchen sink runs cold, but if I run the hot water in the nearby bathroom sink both will be hot. Or, we have a downstairs shower with two shower heads: when only one is on the water is cold, but when both are on it’s hot.

Can anyone suggest an explanation/remedy?! We have a storage water heater.