Your partner has a slight edge, but since the sex with the two men was so close together, you will definitely need a DNA test to know for sure.
If you feel like you are going crazy with the waiting and have big bucks to spend on it, contact Ravgen or the DDC and you can get a prenatal paternity test now.
If not, wait until after the baby comes, when you can get a DNA test for about a tenth the cost of a prenatal test, and you will know then.
(But in any case, you will need to test. You need to put this question to rest forever.)
Testing after the baby comes is the only kind of test a court will accept as legal proof of paternity, so given that you are in your seventh month already, you might as well save the money and do the post-natal test. Talk to a lawyer or the hospital about what constitutes a legally admissible test -- a drugstore test is not going to do it.
Also, whether you test before the baby comes or after, test with both men. One man's negative will confirm the other man's positive, and you will never have to worry about whether the test was wrong.