You and your spouse are the only people who can own the business. There can be no other outside ownership, or you will not qualify for the Solo 401(k).
To qualify for a Self-Directed Solo 401(k), you cannot have any full-time common law employees other than your spouse. You can have part-time employees, but if you hire full-time employees in the future, you will have to stop contributing or come up with another retirement plan solution (maybe a SEP-IRA)
Yes, unfortunately. The IRS can see this as hiding a retirement plan from your employees in another business. If you have employees, you can be subject to "discrimination testing" to make sure that if you are giving yourself retirement benefits or an employer match, that your other employees are also eligible for fair employer match and able to contribute to retirement accounts.
If you ignore this, you could be subject to retirement backpay or legal action from your employees as a violation of ERISA law by enforcement of the Department of Labor. (DOL)
Hold up there. Check if the Solo 401(k) is something you want to do. The Solo 401(k) can be a more complicated product to administer for our customers compared to a Self-Directed IRA, which is simple and straightforward.
If you answered no to all or most of these questions, it might be best to stick with the IRA. You can read more about the Self-Directed IRA vs the Self-Directed Solo 401(k) comparison here.