Hint"Cannot" vs. "can not"
Are they interchangeable? Why?
If you can separate 'can' from 'not', then there's proof they're separate words; but, that's not to say 'cannot' and 'can not' hold separate meanings. They mean one and the same thing, no matter how you write them.
1a. You cannot only....
1b. Not only can you.... (proof that 1c is correct) :D
1c. You can not only....
cannot arose c. 1400 as an auxiliary verb; it's an orthographic (i.e. written) variation of can not. Either or is acceptable. Neither nor incorrect. You can separate them or join them. People usually join them. But I remember being taugh by the Nuns in school not to join 'can' and 'not'. And Webster's 1828 dictionary also mentions "These words are usually united; but perhaps without good reason, 'canst' and 'not' are never united." Those were the good old prescriptivist days.