|All together means as a group:
All together they weighed 40 pounds.
We found the books all together or
We found all the books together.
Note: All together can only be used if it is possible by rephrasing to separate all and together.
Altogether means entirely:
The trip was altogether worthwhile.
Altogether the United States uses 339 billion gallons of water each day.
As a noun, one's birthday suit, presumably all in one piece, uses altogether:
The Style Committee declined to pose in the altogether (meaning nude: note "the").