Officially, any change to a given class feature is an alteration, no matter how benign. Paizo has ruled that you can’t even stack two archetypes that both add class skills, since they both change the “class skills” feature, even though there is absolutely no reason why you couldn’t add more things to the list.
So while herb witch fails to state that the requirement that you select the cauldron hex at 2nd level alters your 2nd-level hex, it’s certainly more of a change than adding options, so I cannot imagine any ruling consistent with the official rulings we have that would allow you to stack these.
The combination of archetypes is, officially, completely character-agnostic. The two archetypes can be combined (or not) based purely on themselves, not who is taking them. They change the class, which is something you choose as a whole when you take its 1st level (and that is, officially, prior to selecting any feats, not that multiclassing as something else at 1st level would help). Taking Extra Hex changes nothing, officially.
However, Paizo rules this way because it is the most conservative of all possible rulings, well, short of “you can only choose one archetype.” The combinations of archetypes in Pathfinder explodes player options wildly, and it is impossible for Paizo to consider the balance implications of every possible combination. And beyond that, it’s also impossible to create fixed and clear and consistent rules for every way in which two archetypes might affect the same feature—adding options to a list might seem safe, but if you make that exception, should you also except things that change options in a list, when they don’t both remove the same option? OK, and then... and so on. It makes the rule more complicated, and creates bigger risk of problems.
And Paizo has the Pathfinder Society to worry about. PFS is a major revenue stream there, and PFS play needs to be clear, consistent, and cannot (as RPGs really should) run on trust between GMs and players. You’re playing with people you don’t know, both GMs and players need “official” support and authority so that they can have reasonably certain expectations about what is and isn’t allowed, what kind of game they will or won’t be playing. So the rule must be clear, consistent, and as low-risk as possible.
Hence the rule we have.
That doesn’t mean it is going to be the best ruling for every situation. It simply is the ruling that has the lowest risk of being a problem in any situation, which is what Paizo needs from its rule. A reasonable GM—in a non-PFS setting—should be amenable to, on a case-by-case basis, allowing exceptions to the rule. But because the rule is the way it is—and this is, no doubt, also seen as a benefit by Paizo—any exception is always solely up to the GM. There is nothing in the rules to give players any room to argue with the GM on their decision. You are, at most, empowered to decide not to play with a GM whose rulings (on this or anything else) are not to your liking.
So yeah, ask your GM. I would allow it, myself. I might make you take cauldron at 4th level, though, which might not be desirable at all, though even if I leaned that way I could probably be persuaded that taking Extra Hex for cauldron is quite sufficient. (I am not sure I’d require either, honestly, but I’d have to review the archetypes more closely.) And another GM might see the only-cauldron-at-2nd requirement as an important balancing factor on herb witch, and not allow you to get around it at all. And a third might just refuse to work with you on stacking archetypes at all. As a player, I’d have opinions on that, but there’s no place for voicing those opinions, unless I felt like it was (as part of a larger picture) a sign that this was a GM I didn’t want to play with.