CAPE CANAVERAL, Fla.—A billionaire’s private SpaceX flight filled its two remaining seats Tuesday with a scientist-teacher and a data engineer whose college friend actually won a spot but gave him the prize. The new passengers: Sian Proctor, a community college educator in Tempe, Arizona, and Chris Sembroski, a former Air Force missileman from Everett, Washington. They will join flight sponsor Jared Isaacman and another passenger for three days in orbit this fall. Isaacman also revealed some details about his Inspiration4 mission, as the four gathered Tuesday at NASA’s Kennedy Space Center. He’s head of Shift4 Payments, a credit card-processing company in Allentown, Pennsylvania, and is paying for what would be SpaceX’s first private flight while raising money for St. Jude Children’s Research Hospital in Memphis, Tennessee. Their SpaceX Dragon capsule will launch no earlier than mid-September, aiming for an altitude of 335 miles. That’s 75 miles higher than the International …