Cut the cakes into half. Fill both mousse in pipping bags separately.
On the bottom layer, line the outside with mascarpone mousse, creating a thick outer. Fill the center with the pistachio mousse.
Put another layer of cake and spread the berries filling.
Put the third layer and cover with mousse in the similar way as before. Finally topping it off with the final layer of cake.
Use the rest of the mascarpone to frost the cake.