>>45455819You choose between the head's first ability or the body's second ability. The stats are a weighted average, with bias towards the Head for HP, Sp.Atk and Sp.Def, and the Body for Attack, Def, and Speed.
ATK, DEF, SPD
fusionStat = 2* (bodyStat / 3) + (headStat / 3)
HP, SP.DEF, SP.ATK
fusionStat = (bodyStat / 3) + 2*(headStat / 3)