I think the reason so many get confused on the copyright issue (I was one of them) is because just about every other picture posted here is some kind of licensed character, some using the toy characters , some not. Then it goes back to "If they're doing it, why can't I"? No one is telling you not to do it, just advising you not to, to save yourself from potential issues in the future.
I would just buy a couple little toys of the characters you want to use and add them to the cake. Is the cake for one child or 2? If it's for 2, why not have one side of the cake decorated along the lines of a Spiderman background (building, spiderwebs, etc.)...a poster here ladyfon did a great Spiderman cake. Maybe something along those lines. Then on the other side, have Dora and decorate in her environment.
If it's for one child, think of a way to incorporate the 2 characters. If you can find a Spiderman figure that looks like he's climbing, why not have Dora on a garden rooftop (decorated using colors and the environment she's from) and have Spiderman climbing up the side of the cake which is decorated to look like a building. Maybe Dora like invited Spiderman to join her for a rooftop bbq! I'm really reaching here, but you get the idea! (I hope!)