Continuity mistake: When Donowitz and Ulmer walk into the theater, they see Joseph Goebbels and his mistress in the box above them, to their right. They would be looking to their right to watch the movie. Later, during the movie, Goebbels, Hitler, the mistress and Zoller are looking to their left to watch the movie, so they would have to be in the box on the opposite side of the theater, where Martin Bormann was shown.
Character mistake: Hicox uses the word "momentarily" in the American sense of "in a moment". In British English it means only "for a moment". An educated Englishman in the 1940s would never have used the word in the American sense.
Continuity mistake: When Shoshanna meets Zoller, there's a strand of hair falling by her left cheek which appears or disappears depending on the angle focusing.
Answer: They're unconnected. As happens often in Tarantino's films, their storylines are completely separate until they intersect at that point.
Captain Defenestrator