What this requires is that the total affections of all partners is greater then 50%. A good tip I got from Fuuinashi on the boards was to find a partner talk to them select the 1st option, travel to a different area and talk to them again selecting the 1st option, keep moving between areas and eventually you're have them at max affection. Each partner at max can give about 7%, so you do need 7-8 partner for the 50% (7 with 100% will give you the 50%, 8 or more just makes it all the more easy to get that 50%).