The answer provided by @DavidD66 requires modification. If your SEP plan uses Form 5305-SEP for you plan agreement as many (if not most) do, you are not permitted to simultaneously maintain both a SEP plan and any other retirement plan (stipulated in the instructions for Form 5305-SEP). You are only permitted both if your SEP plan uses a prototype agreement that allows for both.
There is generally no benefit to maintaining both because, with both, the same employer contribution that could be made to a SEP plan can be made to the 401(k) plan instead.
Also, for someone who is self-employed, the maximum employer contribution is 20% of net earnings (because the employer contribution effectively reduces net earnings from which the maximum employer contribution is determined). Net earnings are net profit minus the deductible portion of self-employment taxes.
