How about decorating half the cake for him and half for her in their favorite hobbies or interests?
I had to do one for male and female both to. I put balloons, one blue clown and one pink clown, both laying down holding strings to all the balloons on it. The writing and border I did in yellow. I didn't put any names on it though because it was for so many people. Although it would have looked less plain with all the names, in my opinion, but they requested it that way 
I have three sets of children who were born a week apart (on different years of course!), and I just do half the cake in one theme and half in another. You could also try one theme that has a side for each of them (ex. mickey and minnie). I know you would'nt use those two persay, seeing as how thery're older and all but I couldn't think of anyone else at the time.
I have three sets of children who were born a week apart (on different years of course
OMG!! What are they and what are their ages!? 
(if you don't mind telling me)
what about the same idea but a 2 tiered cake, one tier for him and one for her! Then you can do their themes on each.. eg handbags stuck all over her tier and tools on his.
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