Can two adverb be used together...according to me here remarkably seems to modify rapidly...
Yes (and 'remarkably' indeed modifies 'rapidly' in your sentence). (However, in this case you could separate them by stating "This behaviour has risen remarkably and rapidly over [the] last two decades" if you want to. Or "Remarkably, this behaviour has risen rapidly over [the] last two decades." All have slightly different meanings.)
An adverb is a word that modifies a verb, adjective, another adverb, determiner, noun phrase, clause, or sentence.
https://en.wikipedia.org/wiki/Adverb [my bold]
Sentences like "John thumbed through the book very rapidly" and "She completed the work the least efficiently" contain two consecutive adverbs ("very" and "rapidly," and "least" and "efficiently") and are grammatically correct. In both cases, the first adverb in the sequence modifies the second adverb. You cannot separate the adverbs in these cases and still maintain correct sentence grammar.
Sometimes placing two adverbs in a row sounds awkward. "She really accidentally tripped" or "He runs extremely rapidly" sound like clumsy mouthfuls. This is because the adverbs all end in "ly." In contrast, the adverb combination in the following sentence flows fine: "Tom is almost always late." These adverbs have different endings and speakers can easily pronounce them.
PS: I don't think that your "This behaviour has risen remarkably rapidly over [the] last two decades" sounds awkward. (I have no trouble getting it out of my mouth.)