In a fanfiction scenario, the interaction could be really complex. Harry Potter's magic is deeply connected to the magical core within him and the magic of the wizarding world. The Scarlet Witch's magic comes from her mutant abilities. When they interact, their magic could create new forms of energy. Maybe their combined magic could be used to create portals to different dimensions that neither of them could create alone. This would open up all sorts of new adventures and challenges for them as they explore these new realms together.
Well, Harry's magic is mainly cast through spells and potions. The Scarlet Witch's magic is more instinctive. In a fanfiction, when they are together, the Scarlet Witch's magic might enhance Harry's spells. For instance, if Harry casts a protection spell, the Scarlet Witch's magic could make it even stronger and more impenetrable. At the same time, Harry could introduce the Scarlet Witch to the concept of wandless magic in the wizarding world, which could further expand her powers and how she uses them.
The magic in Harry Potter is more rule - based and structured, while the Scarlet Witch's magic is more chaotic and based on her emotions. In a fanfiction, they could interact in a way that the Scarlet Witch's chaos magic could break some of the rules of Hogwarts' magic. For example, her ability to manipulate reality might make spells that are normally impossible in the wizarding world possible. Harry Potter, on the other hand, could try to use his knowledge of magic theory to help the Scarlet Witch control her powers better.