I should note the wife's 401(k) plan owns a minority position in the husband's company, less than 10%. We created two different membership classes within the husband's company, Class A (with one vote per unit) and Class B (with nor voting rights or governance rights whatsoever). The wife's 401K owned Class B membership. This effectively ensures the IRS can’t argue that the wife's plan ever had any control over the husband's business. Much like preferred stock there is some type of return on investment to compensate for the loss of voting privileges.
The husband's company is a partnership so not a ROBS. ROBS require investment in a C Corp.
PT exposure is avoided because there is no attribution between husbands and wives.