Under these circumstances, TurboTax does not support using the Maximize function for an individual 401(k). Assuming that you are under age 50, your total elective deferrals to both plans is limited to $23,000. That means that your elective deferral to the individual 401(k) can be no more than $15,500, which you can explicitly enter as your individual 401(k) elective deferral. You can then use the Maximize function for a SEP-IRA contribution to calculate just the maximum employer contribution which would probably be about $4,089 for a total of $19,589 to the individual 401(k). (The calculation for a SEP contribution is the same as for an individual 401(k) employer contribution; SEP contributions are employer contributions.)
