Generally, it doesn't make too much sense to contribute to both a SEP plan and a solo 401(k) plan in the same year. Your total employer contribution would simply be split between the two. In TurboTax, marking Maximize for both a SEP plan and a solo 401(k) plan will result in no more being able to be contributed than marking Maximum for only the solo 401(k). Also note that a SEP plan based on a Form 5305-SEP agreement is not permitted to be maintained in the same year as a solo 401(k). Contributing to a SEP and a solo 401(k) in the same year requires using a SEP plan agreement other than Form 5305-SEP. (Some custodians provide you their own prototype SEP plan agreements to use.)
Having $15,000 of self-employment net profit means that the $14,500 that TurboTax is indicating is for the self-employment elective deferrals and employer contributions combined. This can be accomplished by making it all as elective deferrals to the solo 401(k) since doing so would only bring your total elective deferrals, including those made at your W-2 employer, to about $21,500, less than the $24,000 limit for someone over age 50, so there is no need for you to use the Profit Sharing Keogh workaround. You maximum contribution could also be accomplished by making the maximum employer contribution of about $2,780 and allocating the other $11,720 or so of the $14,500 as elective deferrals. Allocate any less than $11,720 or so to elective deferrals in the solo 401(k) and you won't be maximizing your contribution. If you are permitted to maintain a SEP plan and a solo 401(k) plan in the same year, the $2,780 of employer contributions would be split between the two.
In my own case, I simply terminated my SEP plan at the end of one tax year and established a solo 401(k) plan to receive all of my contributions for the following year and beyond. My investment options and distribution options were the same for both plans, so it made no sense to maintain both since making the maximum permissible contribution to the solo 401(k) meant that I could contribute nothing to a SEP plan (and my SEP plan was based on Form 5305-SEP, so I was prohibited from maintaining both anyway) .
