A sole proprietor (self employed person) filing a Sch C means only one person owns the business ... there cannot be a second person. So is the spouse an employee ? If you have a solo 401K for the sole proprietorship then there can be only one person listed.
Both, because the employer contribution must be the same percentage of compensation for each spouse (for 2018, up to a limit of $275,000 of compensation per individual taken into account and limited to a maximum addition for each spouse of $55,000 plus $6,000 age-50 catch-up per individual).
Also, it's 20% of net earnings. Net earnings are net profit minus the deductible portion of self-employment taxes.
(Critter, a solo 401(k) can cover both spouses who both participate in the business.)