Two yorkies is an awesome idea, I too am feeling the same longing for another (have been for a while, lol) I figure I'll wait until I move next year, it'll put Rizzo at just about 2 and he should be 100% trained (potty wise) by then. I think I'll have to get another boy though, I wanted to do the boy/girl thing, but Rizzo always seems more aggressive around the ladies, and more of a sweetie to his fellow males... Strange considering he hasn't been fixed yet (this month is his month, poor fellow) So I recommend letting him play with other dogs to see which of the sexs he prefers
Good luck!