You have to leave the Velvet Room but say im not done here, then talk to them again, alternatively if you leave the Velvet Room altogether and come back, they will stop you
@Bamtom1234 When you leave the Velvet Room, one of the twins will ask you something like "Is that all you want?" and if you answer "No" then you can walk around the cell. And in the cell there is a small opening (looks like what they use to give food to the prisoners) use that to talk to the Twins and they'll tell you what you need.
I'm.... not actually sure Mako. I think, I THINK he can be registered to the Compendium but I'm not totally sure because I think I recall Izanagi-no-Okami could not be.