How do I give a raise to one better worker without affecting the morale of his friend?

I have two employees and one of them performs better than the other. I want to give a higher raise to recognize performance. At the same time I dont want to de-moralize the other person who happens to be a friend of the good performer. All company rules about pay confidentiality will not work between them, I know. Both of them currently work for same pay in similar roles and have same exp level. How do I manage this?