You do not have to have a different variety, but you do have to have 2 genetically different trees. That means that it can't be a clone of the other tree, such as a root sucker. In the wild, root suckers are the most common form of reproduction in pawpaws.
I have 2 trees that have lost a bit of production since I cleared some sweet gum trees that were shading them. This winter, while they are dormant, I think I am going to make an effort to dig them up and move them.
The stigma(female) of the pawpaw ripens before the pollen is ready, so the flower cannot possibly self pollinate. The natural pollinator is carrion type bugs, such as blowflies. When I was a child, my grandpa would always empty the "pot" around the pawpaws starting in late February, to attract flies to the area. LOL. Your best bet is going to be get an artist brush or a makeup brush and spend a few minutes every week pollinating them manually.