start talkin to himm more than usual. ask him things about him..make it seem like your more interested in him. start flirting with him from time to time, and if he flirts back then find the right time to just ask him.
Me and my friend had the same problem, we both tried to get him before the other, and we ended up have a huge fight/argumen. So.. once it was over, we decided, never date somebody if your best friend...
I'd say that it depends on what you want. You either want both of your friends, and want to keep them, or you want more with your best friend and cant have it so you leave both of your friends.
But...
I probably make the first move.but right after the big question.just ask him or her if it would affect the friendship.Sotake the first move,there shpould be no problem.If so sorry you lost a friend....