Your participation in your employer's 401(k) has no bearing on the amount that you are eligible to contribute to the SEP IRA based on your net profit from self-employment. SEP contributions are employer contributions and the limit on employer contributions at one employer is determined separately from the limit on employer contributions at another employer.
Be aware, though, if the two employers have similar ownership, the two employers might be considered to be a single employer with respect to establishing a retirement plan. I assume that you do not have a substantial ownership interest in the employer with the 401(k).
