Sweetheart, it could very well be the first guy, from all the figuring you have done (none of which is illogical). But when the sex was only two days apart and both events were right around the time you ovulated, you will need a DNA test to sort it out. Since you are at 8 months now, it would be more practical for you to take a test after the baby comes -- you could do a prenatal test now but the cost is about a tenth as much once the baby is born.
Did you happen to have an ultrasound in your 6th or 7th week that gave you a due date from the measurements of the baby? If so, can you put your hands on it and say when the estimated due date (EDD) was? It might not clarify things but if you did have one we might as well check.