It's my brothers and his girlfriends birthday in the same week so I want to do a joint cake for them but just have no ideas that will suit for a boy and girl

What are they into, what age are they?
For a young-ish cake I would suggest bright colours (pastels are a bit femenine) and stars are pretty unisex, you could have a star wire topper and something that has their names on. I would just go for a plain round or square cake, perhaps with stripes down the sides.
Have you had a look in the galleries? See anything that suited?