Another ultrasound done 9/5 states 6weeks0days pregnant.
With your ultrasounds suggesting conception on August 8th or 9th, the question is whether the guy on the 4th or 5th used a condom. If he did, he has a lesser chance of being the dad than the other guy. If no protection was used, you would have had living sperm in your body at the same time from both guys when you ovulated, making it a toss-up whose might have penetrated the egg. I assume it's too early to know if the baby is a girl or a boy?
Really, the only way to know before the baby is born is to do a prenatal DNA test. They are many times more costly than doing a test after the baby comes, but you would know within a week or two. It's done with a blood draw from the mom (her arm) and cheek swabs from the guy(s). Do both guys know that they might be a dad in a few months? Ideally, you and both men could split the cost of the testing three ways. The guys might be willing to test beforehand so the one who is the dad can get emotionally prepared rather than have it sprung on him at the birth.
If this is out of the question (either because you're keeping one guy a secret from the other or because it's too expensive or because someone refuses to test before the baby comes), then a DNA test after the baby comes would be the way to know. At that point, you'd have the authority of the courts behind you if you needed to nudge a balky guy to test.