It’s hard to say without knowing exactly what you need, but possibly zip() or combineLatest().
zip will take both Observable<Milk> and Observable<Cereals> and let you combine them into CerealsWithMilk via a provided function. This emits a new CerealsWithMilk each time you get get both a Milk and a Cereals.
combineLatest is similar to zip except it will emit a new CerealsWithMilk even if just a new Milk or just a new Cereals is emitted.