"All together" and "altogether" are homophones -- words that sound alike, but have different spellings and meanings. "All together," a two-word phrase, refers to things gathered "into a group" or things happening "all at the same time." "Altogether" is a one-word adjective meaning "completely."
Another Altogether Definition
The fun and confusion sets in with a third, informal usage: a person who is undressed is said to be "in the altogether," although the individual may not be in a group, and is, without clothing, certainly not complete.
The Two Homonyms Joined
As long as "all together" remains a two-word designation and "altogether" a completing adjective, they can easily be juxtaposed in any sentence: "The group was all together in one room, and agreed that the room was altogether too cramped." Hopefully, when you use these words, your readers and listeners are all together in agreeing upon their definitions. It would be altogether too confusing otherwise.