I agree with the above posters. Because the sperm from the first guy could have still been around when you had sex with the second guy there really isn't any way to tell without a DNA test.
I would suggest getting to the doctor to get a due date and to see how far along you are. Still those dates are pretty close together so you'll deffinately need a DNA test to confirm paternity. Good luck. *in the same boat*
A DNA test is the only way to know who the father would be. Sperm can live up to 5-7 days in the vagina. I'm sorry! You can get a DNA test while pregnant (which carries risks) or you can wait until they are born.