(as) different as chalk and cheese
Very different from one another or in conflict. Primarily heard in UK. Good luck getting those two to talk to each other—they're as different as chalk and cheese. My daughters are different as chalk and cheese these days—one loves baseball and the other loves ballet.
as different as chalk and cheese (or like chalk and cheese)
fundamentally different or incompatible. British The opposition of chalk and cheese hinges on their being totally different in all qualities other than their rather similar appearance.