Sinkers are called sinkers because they drop more than a fastball. The reason they drop more than a fastball is because they have less back spin. (They don’t have top spin, like a curve, which actually sinks more than a sinker but is easier for the batter to recognize. A sinker can also be thrown harder than a typical 12-6 curve.) Now to your question. Two seam fastballs and sinkers are cousins. A two seam fastball typically is thrown with some pronation, which imparts some horizontal spin that is opposite of the slider spin. The more horizontal spin on the fastball, the less back spin and, thus, sink. A sinker, as I understand it, can be thrown in many ways, each of which imparts less top spin than a fastball. A splitter-like grip can work too. Hope that helps!