March 18 would have been cycle day 11 and March 20 would have been cycle day 13 for you. These are both fertile days and potential days for conception. When you have sex with 2 different partners, both in you fertile period, there is no way to predict or know who is the father. You need paternity testing after the baby is born.
The sperm will still be in you five days later. The second guys sperm does not cause the first guys sperm to die. Either one could be the father. You need paternity testing after the baby is born.