grammar - Is it “James and Paul come” or “James and Paul comes”?

Which is the correct way of writing:

As an appointment, James and Paul comes tomorrow.

Or

As on appointment, James and Paul come tomorrow.

I would also like to know why.

Answer

As appointed, James and Paul come tomorrow.

The verb has to agree with the subject in person and number. The subject of the sentence is James and Paul. The person is third and the number is plural. The third person plural form of the present tense of to come is come (comes is the third person singular).

Note that I changed the introductory adverbial phrase. As on appointment is not an aceptable idomatic phrase in US English (nor British, I believe).

You also could say As per their appointment, but that wouold be somewhat stilted. Probably As scheduled would be even better.