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According to OALD both "all at once" and "at once" can mean at the same time.

I can't do everything all at once

I can't do two things at once.

Don't all speak at once!

These examples give me the idea that "at once" could be used when referring to a countable thing (in the latter example all is countable because it refers to a specific count of people, e.g. a class) while "all at once" is taken when referring to a non-countable thing (of course, everything could possibly be a countable thing).

Are they always used interchangeably or is one preferred in certain contexts?

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There are two different idioms here, and you may be confused. In Don't all speak at once, all and at once are separate, and both modify speak; the normal rules apply to all/both, as Barrie has said. But all at once can also be a single phrase, where all intensifies at once ("When all at once my heart took flight", My Fair Lady). I don't believe there are any rules for when you can or can't use that.

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My intuition is to say that you don't need all when you're talking about only two things. The number isn't large enough to justify its use.

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