Question about electrical service for subpanel in pool house
National Electrical Code says that you cannot connect conductors 1 AWG or smaller in parallel (310.10(H)).  Since it's not likely that the conductors between your buildings are larger than 1AWG, you cannot do what you want.

What you can do, is install a new larger double pole breaker in the main panel. Then install a new set (4 conductors) of larger conductors between the buildings, to feed a secondary panel in the pool house. Then you can feed your branch circuits from the panel in the pool house.

The size of the breaker in the main panel, and the size of the conductors between the buildings will depend on what size service you want to supply to the pool house.