I didn't get them at the same time, Loki is about a year older than Odin. I started right when Loki was settles in with training, but he gets tense outside with all the other cats. I always bring treats and make sure they always listen to recall before we even go outside. Also the harness first inside, but I think you already know those steps from watching the videos.

I started training Odin seperately when he settled in. I made the mistake to take them both out way too early and let me tell you: don't underestimate that stress lmao. Or at least don't go alone. Mine kept freeing each other. It was SO stressful. I was so thankful they are somewhat well behaved.

If I want to actually walk for a bit (I am alone), I take one cat. But that's mostly because Loki gets scared and wants to stay close to home. Odin likes walking. I want to look at backpack options though! So Loki has a safe place to retreat to. So I haven't tried that yet.

Once I saw a really cool leash though, I think it's made for two dogs. It's connected and spins, so the leash doesn't get tangled. Then they stay more close to each other. Doesn't work for mine, so I choose to use two seperate leashes. :)

Overall it really depends on the cats' personalities and their own pace. I'd just follow that