Yes. You can make contributions to both a SEP and a Solo 401K
Plan.
There are limitations on both however based on a percentage of your income you are reporting on the Sch C business.
The employer side of the solo 401(k) contribution is limited to 25% of total business income for the year.
SEP accounts are less well-known and are generally used by self-employed individuals and small businesses. SEP stands for "simplified employee pension," and all contributions are considered to be from the employer. The overall contribution limit is $54,000 for the year or 25% of the employee's compensation -- whichever is less.
Calculating SEP IRA contributions for self-employed individuals is tricky, as the salary of the account holder is calculated after the contribution is made. Not including limits, the calculation is 18.587045% (approximately 18.6%) of net profit
The link below is an article on this very subject you may want to look at. It does have overall dollar amounts for 2016, but it is the percentage of the profits that is impacting you and not the overall dollar limit.
https://www.fool.com/knowledge-center/can-a-person-have-a-401k-and-sep-ira-simultaneousl.aspx
