Most girls LIKE spiderman. But a 1yo isn't even going to have a clue. She's going to like the bright contrasting colors. If you HAVE to have something girly/cutesy... toss in some Mary Jane action :) A little baby fedora and a 'press badge' to get clipped to her.
For the cake you can do a double profile; spiderman looking one way, and Mary Jane looking the other.
It's actually a GREAT boy/girl theme
Same token, if you can get a bunch of cheap disposable cameras for the party (since PP does photography), and some little flip notebooks (since MJ is the writer) you tie them both in again. A mini-scavenger hunt (kids taking pictures of the clues) that leads eventually to the cake would be perfect for the 4yo crowd.
But seriously, you 1yo is going to be absolutely clueless, and probably need a nap from overstimulation. Later on you can be more careful about making it "both" but at this point, it's your son's party. He's going to remember it forever. So if you're "splitting" make it 90/10 or 80/20 at this point.