It won’t be good for both at the same time -most of the time- but if you can source free or super cheap peacock feathers, I’ve found them to be a toy that cats who don’t normally get along all want, and will thus play together better. The great equalizer.

They won’t, in my current three-household-9-cat experience (and so many past but before feathers), play with each other until it’s broken and you ditch it to their devices. They will just play with you (and enjoy the fuck out of it), but pay close attention to other cats playing the same way. And that’s a normal play strategy for cats - they enjoy watching play from cats they know, it gets them revved up for their own turn at play. Cats who get along well take turns playing with you, because they recognize that you don’t play the same way they do, and rarely respond to their play requests like other cats, and are thus a limited, and valuable, resource. They want to give you their energy, because they like what you do for them.

But when the peacock feather “dies”, and gets bent up in weird places, one will chew one end while another plays with the other end. It’s good for everyone. The cat chewing gets dental perks and the cat at the end has a now moving toy to play with.

It isn’t perfect, but you can’t really do anything other than encouraging play between them; they are fully sentient beings, like kids. You can encourage, but ultimately they make their own choices about friends and how to interact with them. And that’s ok.

Other than making boxes into castles (tape or wood glue work wonders), it’s more of an issue of rotating toys like one does with toddlers. If they normally have sponge balls, put them up and give them sparkly Pom balls. If they like strings, use long feathers, wands, or lasers for a few weeks instead. Just rotate their toys and they will keep the energy up while saving you money.