I swear i read somewhere on these forums that it's one entry per category, so I am not an official spokes person for this contest/challenge but i read just yesterday.
They have to be two separate designs for each and you can enter both they just need to be different designs for both 2d and 3d.
You can wait for verification like i am though.
Edit: ok found it you have to click the top post img/picture and it brings you to this:
• You can only submit one entry per category (2D/3D)