My friends hate each other, but I want both of them there at my b-day what should it do?

Invite them both. If they are good friends they will put their disputes on hold in order to allow you to have a good birthday party. If they cannot do that I would consider new friends.

You might want to mention to each of them that you did invite the other and you really would like them to keep in mind that it is your party and you want it to go nicely.

If they are really bad see if any of your other friends will act as minders. I used to do that role, basically it was my job to keep things civil and to remind them that their petty dispute were not to be the focus of the evening.