When you own and control two businesses, for many tax purposes the IRS will accept all of those businesses as if they’re one. Therefore, neither business can break the rule of eligibility.
If your sole proprietorship sponsored your 401k, and you create another business, to continue using your solo 401k and contributing to it, you need to ensure that the other business doesn’t have full-time employees. A full-time employee is considered to be one who works over 1,000 hours a year and gets a W-2. For further questions, please consult your CPA or tax advisor.