at a time vs at once

at a time and at once both are adverbs.

Word	Noun	Adjective	Verb	Adverb
at a time	No	No	No	Yes
at once	No	No	No	Yes

As adverbs, at a time and at once are synonyms defined as:

Other synonyms of at a time include at one time.

at a time (adverb)	at once (adverb)
simultaneously	without delay or hesitation; with no time intervening
	simultaneously