。
She mixed butter and sugar together.
Literal
She [topic-は] butter [and-と] sugar [object-を] mixed-together.
混ぜ合わせる is a compound verb pairing 混ぜる ('to mix') with 合わせる ('to put together, combine'). The 合わせる suffix is highly productive across action verbs to add the sense 'put together, combine, do mutually': 話し合わせる ('coordinate stories'), 折り合わせる ('fold together'), 申し合わせる ('agree, arrange jointly'). と here is the noun-listing 'and' — exhaustively connecting items into a single conjoined object. Mixing butter and sugar is the foundational creaming step of countless baked goods; Japan's contemporary baking culture inherits this from European cake-making traditions imported during the Meiji and Taisho eras.