You can do this but you should also relinquish all voting rights of said companies. When companies like Blackrock, State Street, Fidelity, Vanguard, etc all sit on the same boards, you're going to get decisions being forced that may not be good for the company, workers, customers, or country. Doubly so when they're sitting on two competing companies boards.

You can allow mutual funds or ETFs or whatever similar instrument, but they should not be allowed to have a vote or say in the company.