Your challenge is to create 2 cards that work extremely well with each other. There must be a way that they influence each other. You CAN make a card that works well with an official Magic card that's already been made, as long as you supply a link to that card.
You cannot create paradoxes or infinite loops (You can't have a card that says "whenever you scry, Bolster 1" and another that says "whenever you Bolster, scry 1"
You cannot have two cards that directly influence each other just because the other exists. For example, you can't have a card named Zombie and another card that says "creatures you control called Zombie get +20/+20.
Try to keep it serious. Humour in the flavour text or name is fine, as long as the card is mainly serious and realistic.
Finally, try to make combos that you would use in a game. You could even print out the cards and have a game with them (and a full-sized deck, of course) to see how well the combo works.