Should I even try to feed separate foods?

Okay guys. You know me. Booboo and Chickster. They were both on C/D, then Boo got diagnosed with CRF. Chickster needs to get off the C/D because she's getting too fat and doesn't go outside. I don't want her on the C/D if boo's not on it because it's 17 dollars a bag and if Boo can't have it it will just cause more issues.
But I've made another issue. I bought a bag of Purina One for Chickster, which she liked at first, and Royal Canin Mature 27 for Boo (One of the top 4 lowest Phos dry foods, because he likes a little dry food from time to time and I want to give him the option of having it)
Now Chickster wants his Royal Canin and nothing to do with the Purina! I was trying to get her to jump up on the table to have hers, since he can't get up there, and he would have no clue there was anything different up there. No go. She knows from Mom that she's not supposed to be up there. If I leave it on the floor Boo will eat it. Also a no go.
So, can I feed Chickster the Royal Canin Mature 27? It says it's for cats 10 years and older, but if I can feed this to her, then I can combine the money for separate bags and feed them together again, which while they act like they can't stand each other sometimes, I've noticed lately they want to be together a lot more. I don't know. I'm so lost! We've always free fed and never had to face issues like this before. I want to do right by them both. Thank you for any advice.

Something I have found feeding several cats different foods, they always want what the other has - even if its a prescription food you can't get the one who needs to eat it to actually eat it.

To feed mine separately, I ended up having to do meals and put them in separate rooms but they were real prescription foods not just senior where I would not worry so much if they snacked on each others food.

Royal Canin 27 has a lot of carbs so if you can help it, I would not feed it to her

Thank you Icklemiss, at least now I can continue separating them and have more determination about it.

Please do not mix them... keep them separate ... RC may be high in carbs but it is one of the better seniors with all things in the right proportion for a senior cat( of course IMHO seniors should be on wet( canned, homemade or raw))

Purina One makes some different flavors. Did you try a different one, since Chickster is now turning her nose up at it? Maybe you can get a flavor that is more in line with the Royal Canin.

No, I am not mixing them. They are separate. Though both of them are pretty upset with me over this and I haven't seen either one very much today. Chickster will eat the Purina One, but if she sees the Royal Canin sitting there she'll eat that. (That happened earlier today) My friend told me that I just should just stick to it, like she did, and she'll eat it when she figures out she's not going to get anything different. Will just have to keep an eye on them. And BOo is eating more wet than dry, but he likes to crunch on five or six pieces of kibble now and then. He doesn't eat a whole cup or anything like that of the dry.

Early on with my cats I was faced with the dilema you face now. When Max was alive, he could eat an kind of food, but Speedboat was diagnosed with urinary problems. I fed them both the prescription food at that time. I quit free feeding them and they got a prescribed amount two times a day. Then Muffin was diagnosed with urinary blockages, etc. At that time, I started separating them. I would put them all on the porch. Muffin and Speedboat were put in a cat carrier to eat their meals and Max went back to regular food. It was too expensive by then to feed it to all three if I didn't have to. So they ate two times a day and got no snacks in between. It worked. I still have to do that now because I still have Muffin eating the prescription food and have added Mattie to the family. Mattie eats regular food. I wouldnt know how to act doing otherwise, because I have had to feed cats separately so long.