all at once vs all together

all at once and all together both are adverbs.

Word	Noun	Adjective	Verb	Adverb
all at once	No	No	No	Yes
all together	No	No	No	Yes

As adverbs, all at once and all together are synonyms defined as:

all at once (adverb)	all together (adverb)
suddenly	all at the same time
all at the same time	used of a group whose members acted or were acted upon collectively and when `all' and `together' can be separated by other words