Take that estimated
due date home, and count back from it on a
calendar 266 days. That will tell you an estimated conception date.
With the two guys this close together, you might not get as clear an answer as you would like (for example, on the 4th, both guys would have had living sperm in your reproductive tract, though the first guys' sperm wouldn't probably be as plentiful). But if your estimated conception date falls on the 7th or later, that does suggest only the second guy.