Inseparable Friendship

When Inseparable friendship enters the battlefield, select two creatures you control. Whenever one of those creatures or both deal combat damage to a creature or player, it or they gain +X/+Y until end of turn, where X is the power of the other selected creature and Y is the toughness of the other selected creature. This efect lasts only as long as you control this enchantment.
Nothing can separate them. Obviously...