I have these two friends that don't get along and I want to invite them both to my birthday.
Why do you want to invite them both?
I do what I want, I'm the boss of my birthday party.
If they are both your friends and you want to invite them both, you should be able to do so. Hopefully, they understand that it is your day and they behave well even if they don't like each other.
I agree and I'm going to invite who I want.
It's your birthday and I would hope that both of these people will make an effort to get along with each other, even if only on your birthday, as it's the right thing to do. I understand that you want them both to be a part of your day and hopefully everyone will have a great time!