Antitrust laws don't work because they're subjective and are enforced by political appointees.

The simpler solution is a tax on scale -- a graduated corporate revenue tax, aggregated across any group of entities which meet the common control [1] criteria. Then it's just a tax, and you simply have to collect it. Very little wiggle room.

If splitting your company in half wouldn't impair any of its lines of business, the CEO has a powerful financial incentive (lower tax rates on the two halves) to do so.

[1] https://www.law.cornell.edu/cfr/text/26/1.52-1