Given the long life of sperm in women's reproductive tracts, either guy could be the dad. From the 14th to the 19th you would have had sperm from both guys in your body.
Your positive pregnancy test May 2 suggests April 19 or 20 was the latest point where you could have gotten pregnant. It doesn't help rule out or pick out anything earlier. Pregnancy tests are effective from a couple of weeks after the unprotected sex. They don't rule out sex that came before that point.
If you have not had an ultrasound yet and can get one, you might get one right away. It's about the only thing that could provide even a ghost of a clue about this unless you want to spend the big money on a prenatal DNA test with Ravgen or the DDC. If you wait, it will be too late for an ultrasound to clarify anything (if it even can), but if you can get one this week, it might help to pinpoint the date. Your problem is, though, that if it pinpoints the date to one where you had live sperm from both men in your body at the same time, it won't help.
If you do get in for an ultrasound scan this week, tell them your periods are irregular and you don't want them to use your last period to tell you how far along you are, you would like them to use only the measurements of the embryo to get to an estimated due date. (Don't just go home with an estimate of "weeks pregnant" as that will not help.) When you get the estimated due date from the measurements of the baby, take it home and put it into a conception calculator. That will give a best guess at an estimated conception date. Maybe, if you are super lucky, it will point to before the 14th by enough of a margin that you can use it to make an assumption about who is the dad. I'm afraid it will just point to the week of the 14th, though, and that won't give you the clarity you need.
It looks like a DNA test is the only way you will know for sure.