Are there two adverbs in the following sentence?:

It usually rains heavily when I'm on holidays.

Is it possible that there are two adverbs?

Yes, heavily defines the verb rains, and usually defines the adverb heavily.

Thanks! I have learned something new today about English.

Actually, there are three adverbs in this sentence:

It usually rains heavily when I'm on holidays.

(1) usually (adverb of time)
(2) heavily (adverb of manner)
(3) when I'm on holiday (adverb of time)

Don't forget that adverbs can be adverbial phrases too.