To do this, general opinion is that you can set up a holding company for the separate companies and make the holding company the adopting company for the Solo 401k. A holding company is usually a company that doesn’t do any business with anyone, it just owns other companies. The trick to making this work is of course to have the other companies which would be called, “Subsidiary companies” – the companies that go out there and do business and are owned by the holding company.
Those need to be passed through entities; meaning that they don’t pay taxation at their own level, they just pass through the earning up to the parent company. So LLCs are pass through entities, and if you use LLCs, then one other benefit you’ll get is that you’ll have the subsidiary company structured as an LLC and it only has one owner or one member which would be the parent company. It would be a single member LLC. If you have a single member LLC, it doesn’t even have to file a tax return in the first place. Single member LLCs the IRS instructs to be disregarded for tax purposes. For further clarification, please consult your CPA or tax advisor.