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  1. You should have known that he is quite adept at what he does and that he has the best conversion rate.
  2. You should have known that he is quite adept at what he does, and that he has the best conversion rate.

Which of the above two sentences is right? I believe it is the first one, as it appears to be a case of compound object and hence should take no comma.

Also, if there can be a case without a compound object and the second sentence has an 'and that' to start off with, what should I do?

Is it similar to 'so' and 'so that'?

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The advice of the late Professor Trask http://www.sussex.ac.uk/informatics/punctuation/comma/joining is:

Use a joining comma to join two complete sentences with one of the words and, or, but, yet or while.

Your example contains two complete sentences. One is 'You should have known that he is quite adept at what he does' and the other is '(You should have known that) he has the best conversion rate'. So use a comma before and to signal the start of a new clause. The purpose of punctuation is to help the reader understand the grammatical structure of a sentence, and a thoughtfully placed comma will do just that.

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    Each of the clauses that he ... are used as simple direct objects. Your logic would lead to You should have known Mary, and John. Two sentences *could be crafted from that. In general, commas do not separate a simple series of two things, even if those things are clauses. You could argue that the comma is used to create a natural break in the flow of a long sentence and to emphasize where the second clause begins, but not because they are independent clauses. – bib Feb 4 '14 at 13:00
  • There’s a difference. ‘that he has the best conversion rate’ is a clause. ‘And John’ isn’t. – Barrie England Feb 4 '14 at 13:24
  • I thought along similar lines too, but then the fact that 'he has the best conversion rate' is just another object of the sentence created the confusion. That makes it two different views from Bib and Barrie. So what is it now? – user64507 Feb 4 '14 at 16:29

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