For starting having 2 surviving sons instead of 1 would make Alois Hitler less willing to press hard his heir(s) into following his steps. He wanted Adolf to be a customs agent like him, and became something desperate when Adolf (his "last chance") turned to be a bad student that disliked that job, and at the same time developed some stupid love for art. Alois ended being an abusive father that hit his sons and wife, and Adolf grew hating him and all what he represented. A pair of twins could/would change that, and the result would be an Adolf with a different personality.
At the same time, being two would make the Hitlers more popular in school. Each would have his brother as a base to start his relations with the other children. This was a problem for Hitler in OTL, as his family moved from a town to another very frequently. Both could help each other in the studies and make the couple more successful too (Hitler was pretty bad outside Geography, IIRC) and in turn Alois would be more proud of them, even if none was willing to follow his steps. With some aceptable marks, none of the Hitlers would be so angered with the teacher's pet, Ludwig Wittgenstein. You know, that rich jew kid that was promoted two grades because of his extraordinary marks, and that became one of the most prominent philosophers of the 20th century. Little Adolf was in the same school at Linz (perhaps even in the same classroom for a while) and hated him in OTL.
Hitler could even decide to not go to Vienna after his father's death, or to not join the Arts School if he went. Which would have evident ramifications in his future character. Etc.