Translation:They mix water and juice together.
Same doubt? Also because you (plural) could take turns mixing things in the same bowl/bottle/? but you can’t mix two things without them being together so I think it’s redundant (in this context—maybe it’s necessary in any other context where you need to precise that there are two separate bowls, for example one with water and one with juice, and for some reason someone has to mix them both but separately).
It's not really redundant, because it implies that neither ingredient is more important than the other - you aren't "adding the water to the juice" or "adding the juice to the wine".
Words like "blend", "mix", "stir", "combine", "add" and "join" are often qualified with "together".
Just as in English, it is technically possible that le chéile/"together" could refer to the people doing the mixing, but that would be unexpected, unless the context clearly prefers that interpretation.