At once and All together

At once adverb - Without delay or hesitation; with no time intervening.

All together and at once are semantically related. in at the same time topic. In some cases you can use "All together" instead an adverb phrase "At once", when it comes to topics like simultaneously.

All together adverb - Used of a group whose members acted or were acted upon collectively and when `all' and `together' can be separated by other words.

At once and all together are semantically related. Sometimes you can use "At once" instead an adverb phrase "All together".