Yes it is, technically, possible to have twins (non-identical, obviously!) with two different fathers. However, it means you have to have released two (or more) eggs rather than the normal one a month (you're born with all your eggs, so they're released, not produced as such) and have sex with two men in reasonably quick succession.
So, sperm from man 1 could fertilise egg 1 and sperm from man 2 egg 2. Although, 1 man could fertilise both eggs.
Fertilsation though is not the pregnancy as such, as then the embryo needs to attach to the womb (ectopic pregnancies are when the embryo fails to migrate far enough and starts to grow in the fallopian tube). Attachment does not automatically happen, there's a window of opportunity for this to occur, so people could have an egg fertilised, but you won't be pregnant until it is attached.
Once the embryo has attached pregnancy is established and the embryo will grow. At this stage, ovulation stops so you can not get pregnant again, as there is no egg for the sperm to meet. After four weeks of pregnancy, a mucus plug is fully formed and seals the cervix, preventing sperm access.
However, there is a condition with two wombs...
If you are already pregnant, then you must be seeing some healthcare professionals for check-ups, so ask them any questions you have. They are there to provide answers and allay your fears as well as the prodding and poking stuff.