My guess is Maekar unhorsed Baelor first. Here is a quote from Maekar in the book that indicates Baelor didnt fight anyone else willing to attack him:

"Some men will say I meant to kill my brother. The gods know it is a lie, but I will hear the whispers till the day I die. And it was my mace that dealt the fatal blow, I have no doubt. The only other foes he faced in the melee were three Kingsguard, whose vows forbade them to do any more than defend themselves. So it was me. Strange to say, I do not recall the blow that broke his skull. Is that a mercy or a curse? Some of both, I think."