My husband has a business with full time non-owner employees. I do not work in my husband's business and I do not own any of my husband's business.
I own and operate my own PLLC.
My husband's business does not interact with my business.
If I establish a solo 401k under my business my husband may want to participate in my solo 401k plan because my husband might be performing some work for my business.
ANSWER:
You may be able to set up a Solo 401k for your separate business but your husband can't participate in the Solo 401k (even on account of work performed in your business).
Typically, your spouse’s ownership of another business would be attributed to you such that if the other business has employees it would prevent you from opening a solo 401(k). In order to avoid the application of these attribution rules (And assuming that there is no interaction whatsoever such that the affiliated service group rules would not apply), your husband would not be permitted to have any direct ownership nor even participate in your business. See the discussion of the attribution rules as they applied to controlled groups at the following:
https://www.irs.gov/pub/irs-tege/epchd704.pdf