From a typical definition of an adverb one can see that a verb is not the only part of speech, potentially modified: Adverb is a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, etc. So our case is not necessarily about modifying two verbs; with some effort we can find only one:
Парк [находится] совсем не здесь.
The verb находится is omitted but is kept in mind, so let's try to create an analogy with a similar English version. The sentence allows for a grammatically close translation form, with the verb 'is' replacing the omitted Russian verb:
The park is definitely not here.
не здесь (=not here) is an adverb (здесь is местоименное наречие in Russian grammar) with a negation particle, this pair of words expresses a relation of place and modifies the omitted Russian verb (находится, represented by 'is' in English); the other adverb совсем (definitely) doesn't modify a verb, it modifies the adverbial phrase не здесь (не здесь -> совсем не здесь; not here -> definitely not here) and expresses a relation of degree.
So, one adverb (совсем) here modifies another one (не здесь, adverbial phrase with negation) and defines how much the latter one modifies the verb which is only meant in Russian version.