Well, if a doctor and two directly interested guys, plus a period and early ultrasound evidence, point out that there is no question at all about who the dad is, it's hard to imagine what someone you don't know on the Internet can say to convince you. (I will note that you have company, though ... we hear from women all the time whose dates and evidence leave no possible doubt about paternity, and they are freaked out anyway. One woman had a lover who was a month off the date of conception and had had a vasectomy much earlier, and she not only got TWO prenatal tests but also had him get his vasectomy checked (!), and then fretted through the entire rest of the pregnancy that all the tests were somehow wrong. Hint - don't do that.)
You don't say if the test you want to do is prenatal or post-natal. I'd say it would be entirely foolish to waste your money on a prenatal test. They cost ten times the amount of a post-natal DNA test, they usually aren't admissible in court to prove paternity, and you already know the answer anyway. You couldn't have had an ultrasound at 5 weeks 6 days and some doctor read it and miss the fact that the baby was actually in its ninth week.
However, (and the following two paragraphs only apply if you are not married, so skip them if you are married to the second guy), it's a good idea to get a DNA test once the baby is born. (The fact that such a test will put your irrational worries to rest is not a bad side benefit either, though don't tell the guys this is why you want it or they will be confirmed in their growing suspicion that on this topic you are cray cray.) Though there is no question who the dad is, you would do a DNA test because it is important for the baby's legal rights.
If a married woman has a baby, the law assumes her husband is the father, and it is a difficult assumption to break. But for a baby born to an unmarried woman, a DNA test needs to be done for the court to determine paternity legally. Your doctor, or the clerk of the family court in your area of jurisdiction, can tell you what lab(s) they accept for DNA tests for legal determination of paternity. This is very important because your child has legal rights, and in the event that the child will have future needs of its father you want everything to be in order. So a DNA test is a good idea (and if you want, you could do it with both guys, but it's likely the first guy will say forget it and he would be justified in doing so!), just don't waste the money on a prenatal test. It won't give you legal proof of paternity, and that is the only reason you would need a DNA test on file at all.
So, stop fretting, don't be foolish in a way the guys can see, congratulations, and good luck!
Annie
And my due date is April 11th