Yes, you're able to have both. See this explanation for example.
Finally, if you have a 401(k) through an employer and have a SEP IRA for your self-employment income, each contribution is treated separately. You can defer up to $18,000 of your salary into your 401(k) and your total contribution is limited to $53,000 ($59,000 if over 50) after your employer contributes, as discussed earlier. Since your SEP IRA contributions are considered to come from the employer, none of it counts toward the $18,000 salary deferral limit, so you can contribute up to $53,000 or 25% of your earnings, whichever is less.
The amounts here are a bit old; 19k is now the max for your own contributions, and 56k is now the max for the employer contributions.
So in your case your limits are your total W2 income (up to 19k max) in your W2 employer's 401(k), and 25% of your contracting income, or $11,750, to the Solo 401(k) or SEP IRA (both of which use the 56k max).