A self-employed retirement contribution is not a business deduction, it's a personal, above-the line deduction. As such it should not be on either of your Schedules C but should instead appear combined on line 15 of Schedule 1.
Examine the Keogh, SEP and SIMPLE Contribution Worksheet to see the calculation of each of your self-employed retirement deductions. (Of course if these contributions were to a designated Roth account in your solo 401(k) plan, there is no deduction.) If you are using the online version of TurboTax you'll need to pay for TurboTax before you can download the pdf with all worksheets.
