Based on how many aristocratic societies are, I don't think that's possible. Being backed by different powerhouses is one thing but combining said powerhouses is another. Besides, most nobles are humans in their greediest state so they wouldn't accept someone having so much power (when they're not the royal family and has no ties to said royal family and is also not easily exploited), especially a woman because of how misogynistic most aristocratic settings are.
it's not possible to combine two different houses. the only way for her is to give birth to two children and give them each cuz it would be a pity if she cant get both fr. nonetheless, bc both house dotes her, she is alr the most powerful person.
craaaaaaazy idea but couldn't she inherit both titles, unify them and basically be the most powerful person in the whole empire?... just thinking