i may be late but this is what i was thinking and im not fully sure of what u meant...

1. Move watermelon layer over avacado layer
2. Ctrl click on avacado layer (which selects the all of the avacado [aswell as the background so make sure you delete the backgrounds around the layers]
3. press shift + ctrl + i (to inverse the selection)
4. click on watermelon layer
5. Press delete

And there u go u have a watermelon in the shape of an avacado
But thats a damn weird project for bio, wouldn't that be a computer art project ?