Your solo 401k is a separate entity from you, @Cliff T. It is a trust. Just like a corporation or LLC, it can lend money, so your trust's name, not yours, would appear as the lender on your loan documents. This process is super easy after the first few loans, and we've been lending both our after-tax and Solo 401k money, secured by local house flips, for years. You, as the trustee, are allowed to manage the loan, but you cannot take a dime from it until you begin taking disbursements – typically at retirement. Since the money is either tax-free (Roth Solo 401k) or tax-deferred (Traditional), your balance will grow very quickly, especially at hard money rates.
Sorry if I confused you, but I was responding to your question about whether you can personally partner with your solo 401k as co-lenders. In this case, you cannot because you are considered a disqualified person. Disqualified persons include you and your lineal descendants (mother, father, grandparents, sons, daughters, grandkids, etc.). Lateral relatives, such as brothers and sisters, are fine.
The other issue I mentioned is a bit more technical and has nothing to do with who the lender is. If there is more than one lender on a deed of trust, then the loan needs a security exemption. Fortunately, California provides this if a CA-licensed broker is used, who follows the rules for originating fractionalized loans. You will need a broker anyway if your interest rate in CA is greater than 10% APR.
Your solo 401k aside, I strongly recommend that you SEE A LENDING LAWYER before you start lending money. Lending law is complex and varies from state to state, so it is essential to have a lawyer who is familiar with the regulations. You don't know what you don't know, Cliff, and you're not going to get it all here.
Last, do not get your loan documents from a title company. Do not. Title companies are insurance companies. They employ many lawyers who work for them, but these lawyers do not represent you. Instead, hire a lawyer who specializes in lending to draft your loan documents and educate you. As a courtesy, you might get a simple note from a title company and a short-form DOT, but this is not even close to a complete loan package and will not fully protect you or your 401k. With all the automation and competition among lawyers lately, prices have dropped for loan documents. Understand that your borrower will typically pay for these anyway.