Well, sweetheart, you are going to have to make plans to do a DNA test when the baby is born.
If your due date had been based on a very early ultrasound, such as your 6th or 7th week, it might possibly have been helpful to try to split the difference between the guys. Unfortunately, at 28 weeks gestation, the margin for error is too big for an ultrasound to tell you when conception might have been.
Also, even with the second guy pulling out, it is possible to have exposure to sperm from the sexual event. (This can happen from sperm being in the guy's urethra already, and then coming out into the woman's body on his pre-ejaculatory fluid.) "Pulling out" is not really a method of birth control, it's more of just a justification the guy uses to not have to wear a condom.
Since the dates of sex with the two guys are so close together, there really isn't even a way to guess or to say one had better odds than the other. All you can do is get a DNA test with both possible dads when the baby comes. If they are balky, a judge can order it.
I'm sorry there isn't a clearer answer at this point. Not long now to wait to find out, though.