You'd need to actually draft a law rather than write it on a napkin and there would be armies of very well paid lawyers trying to avoid it, but that's common to

You'd need to actually draft a law rather than write it on a napkin and there would be armies of very well paid lawyers trying to avoid it, but that's common to any attempt to break up concentrated power. In that context it wouldn't be all that hard. You'd need to figure out some definitions for common ownership, joint enterprise, and so on. A lot of those laws and definitions already exist. It's only really in the last few decades that we've abandoned antitrust law but we have successfully done things like this in the past.