If I add a second teabag to a cup, how much the concentration of tea in the cup increases?
It will nearly double. A mug of boiling hot water can take at least 10 teabags before saturating. As water cools down, the strong solution becomes less clear due to a sediment forming, because cooler water can't dissolve as much as hot water. This effect is behind the tea preparation method where a very strong tea made in a small pot is diluted with water before drinking. Bitter tea ingredients are less soluble thus yielding a better flavor with this method.
Tl;dr It depends.
The water will dissolve 'tea flavor' until either the tea bag runs out or until the water is saturated with flavor. Hotter water can generally dissolve more. The more water in the cup the more likely the tea bag will run out. If the water is saturated, the second teabag will have no effect.