Your ultrasound was late for this purpose, not early. If you are trying to use an ultrasound to judge when you got pregnant, a 7th-week ultrasound is much more precise than a 13th week ultrasound. All babies begin as one cell and develop at a known rate for a while, but after a while some develop faster and some develop slower. The margin for error at 7 weeks is only plus-or-minus a day or so. The margin for error at 12 weeks is plus or minus 7 days. The margin for error in your 40th week is 3 weeks. So the date info you got from a 13th-week ultrasound has to have the caveat added that it is such and such a projected due date but +/- 7 or 8 days.
If you did ovulate around the 8th (my calculator says that instead of the 9th) it wouldn't matter anyway, because you would have had live sperm in your body from both men at that time. Sperm lives in the woman's reproductive tract 5 or even some research says 6 days. So the first guy's sperm was still there when you had sex with the second guy.
Neither guy is ruled out. DNA test will be the only way to tell, get it with both men.