I'm not very good at coming up w/ ideas for cakes, but just the other day, I saw a "double duty" wedding cake. It was a bride and groom cake in one. Very unique idea and I tho't it was cute. It may work for this, as well. I'm interested in seeing what some of the pro's say.....
Good luck!!!
I would do three small tiers like 4, 6, 8. Or depending if you need more cake. And do one tier with angry birds and the boy's name and another tier with hello kitty and the girl 's name. And then maybe incorporate something for the two of them at the top tier.
Why not two small cakes side by side on a cake board?
Or one larger cake divided with a curving line and each part decorated separately.
Search "storybook" on galleries and you will find several sheet cakes that look like an open book with each page being a different story. That might be something you would want to consider.
Don't know how many people you are needing to serve but the idea of 2 smaller cakes is good.
Or put hello kitty in the "cage" that the angry birds have to crash to get out! 
I know you'll do well!!!!! Good luck!
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