To answer the specific question: No, there is no restriction. You can use
〜たち with two people but you aren't exactly saying "two people", you are saying "groups of ~"; the definition given by JDIC being
〜たち is used to refer a group and this group could very well include only two "animate objects" (or groups of "animate objects"); the number is indeterminate.
To specifically address what Sjiveru said: you would never call your own parents
親達. His point is valid in that
親 means parent/parents and if someone wanted to refer to their own parents as a group/couple, they would just say
Referencing weblio for 親達 gives the following example.
The parents are made happy by their son completing his education.
親たち is used because the
親たち is not the speaker's own parents. You could just as well use
親 here but that would come off as, at least to my ears, as more direct and less polite.
As to why you would not use
親たち with your own parents, I don't have a concrete explanation. If I heard a native speaker refer to their own parents as
親たち, to my ears it would indicate some weird distance between the speaker and their parents. If you, as a non-native speaker, said
親たち in reference to your own parents, it would be understood by the context but it would be unnatural in the same way that non-native speakers overuse
あなた in situations where the subject is already understood; they know you most likely have two parents, you don't have to add
たち and (over) explain it to them.
Referring to mirka's comment, I also don't have a concrete answer. I suspect the reason is because
子ども does not have a "strong implication to mean a father + mother" like
親 does and so whether or not
子ども is being used as a singular or plural noun is less easy to determine from context. Also, in a lot of cases the specifics of
親 doesn't matter. If I tell my son he can't do something, he'll say to his friends:
親 means father, mother, or both isn't important. His
親-figure said "no" and that's all that matters.
My guess is that with
子ども, in addition to the noun's person being harder to determine from context, whether or not it is plural matters more when understanding the sentence.