I got this ending without ever using them together in my party or using any sort of book items on them. I simply viewed the private action in the jewelry shop in Eckdart where Ronyx buys earrings for Ilia.
Earlier in the game I saw a private action in Haute with Ronyx, Mavelle, Millia and Ilia which triggered as soon as I had Millie back in the party. A bit later I got a PA in Tropp with Ronyx and Ilia. These two PAs are probably required for the Eckdart one. The Eckdart PA also wouldn't happen until before I was about to enter the demon gate. Just make sure you go back and find the Haute and Tropp PAs as soon as you have gotten Millie and Mavelle and you should be good.