ANSWER: It is prohibited for a solo 401k to invest in the solo 401k owners' business. However, the solo 401k owners may take participant solo 401k loans and invest the funds in the business since they would no longer be deemed solo 401k funds. Of course, the solo 401k loan(s) would need to be paid back over a 5 year period.
Solo 401k Loan Facts
Solo 401k Prohibited Transactions
401k Business Financing