I'll quote what my NOAD says:
USAGE - The merging of all and right to form the one-word spelling alright is first recorded toward the end of the
19th century (unlike other similar merged spellings such as altogether and already, which date from much earlier). There is no logical reason for insisting that all right be two words when other single-word forms such as altogether have long been accepted. Nevertheless, although found widely, alright remains nonstandard.
So, although alright is "older", altogether is considered to be standard.
Plus, as the OALD states, altogether and all together are not synonyms:
Altogether and all together do not mean the same thing.
- Altogether means ‘in total’ or (in British English) ‘completely’: We have invited fifty people altogether. ◇ I am not altogether convinced by this argument.
- All together means ‘all in one place’ or ‘all at once’: Can you put your books all together in this box? ◇ Let’s sing ‘Happy Birthday’. All together now!
And, as @Matt Ellen reminded me (us), not even all right and alright are synonyms, see his comment.
Other terms are already, as mentioned above, and about the
al- prefix, the Oxford English Dictionary, says:
obs. form of all, retained in comp. in albeit, almighty, almost, alone, already, although, always.