It’s possible that Yuca simply doesn’t enjoy puppies. And that’s NORMAL and perfectly okay! Puppies are annoying. Your best bet is to put a LOT of attention on working Coco’s brain more. A tired puppy is a good puppy, and the BEST way is to work their brain. This would be obedience training, beyond basic sit/stay/down. I know group classes aren’t really a “thing” right now, but I’d look into classes through Denise Fenzi’s dog sport academy: https://www.fenzidogsportsacademy.com/
Maybe work Yuca while you’re at it. It’s a good way to keep her engaged, and working them together can help.
How did you introduce them? Have you talked to Nomi’s obedience trainer for ideas? This sounds like it COULD be generalized resource guarding, and it might mean she needs more confidence.
Good morning-
I think starting over from scratch might be helpful. Complete re-introduction for both dogs. A trainer will be helpful as well- even if your dogs already have obedience training. It just helps establish some routine as well as pack order. I am a firm believer in walks. They are another fantastic tool that help keep things in order. And personal space vs. supervised play will be helpful also. Setting up a place for your 3 yo dog to go for a break (crate, bed, etc.) that they know is for them only can also be very helpful. When our senior dog was still with us, if he went to his bed- everyone dogs and humans alike- knew to let him be, he needed a break. Our younger dogs do the same thing, and it really helps keep things calm. I hope you find this helpful. Again, a trainer who can see things firsthand will probably be money well spent and prove very helpful. These are just some things that help in our house. Best of luck!!!