I have an scorp. I like to hire full time employees but still want to keep my solo 401k plan. I am thinking of starting a new business and hire employees at the new business so I can continue my old business with me as the only employee and keep the solo 401k. Can someone confirm if this is legal or are there any laws that prevent opening multiple businesses to keep the solo 401k plan?

What you propose is perfectly legal, as long as the businesses are completely separate and fund for the businesses do NOT inter-mingle.