Let me point out that Lee Harvey Oswald was the only person on the face of the earth who can be identified as being on the scene during both murders
Take that little gem to the (blood) bank
I'll add that it seems to me that someone who expects help, someone who is part of a larger conspiracy involving powerful elements or figures wouldn't shoot a police officer. Not in broad daylight, not in such an obvious manner.
Oswald's shooting of Tippit was the act of a desperate person, someone essentially fighting for his life. It's why he didn't worry about hiding the shells.
It's impossible here to prove a negative: that is prove that Oswald didn't have help. But if he did have help they must not have been people who he thought could free him if caught. Otherwise why blow away Tippit?