I can't think of any linguistic term that would really fit the bill.
Productivity may be relevant here, but it doesn't address everything that you're talking about.
As I see it, your two examples involve very different operations. In the case of
-er, you're talking about the application (or misapplication) of a rule. Even a small child would understand that
-er is added to the end of a word — I don't think anyone would say "er big" instead of "bigger" — but the child may not yet have mastered the finer points of the rule's application and may therefore produce forms with a duplicated comparative, such as the aforementioned "biggerer" or something like "more taller," which is how I described a Big Mac when I was three years old. Non-standard forms of this sort can also be used deliberately with humorous intent, as in the case of Little Caesar's Extra Most Bestest Pizza.
The "twoness" of the word
both, on the other hand, is not a matter of a grammatical rule being applied but rather part of its core meaning. In other words, your example of "both
big, bigger, and
biggest" is incorrect in the same way that "all three Beatles: John, Paul, George, and Ringo" would be.