If you had two partners 2 weeks apart how can tell who are pregnant by?

It is not possible to tell who you got pregnant by from having sex with two partners two weeks apart. Sperm can live in the female reproductive tract for up to 5 days. So if you had sex with the first partner, the sperm could still be viable when you had sex with the second partner. In this case, it would be impossible to know for sure which partner is the father.