If I open a Solo 410K and a Roth 401K, are the contribution limits separate or combined?

I am self-employed and have a SEP IRA. I am thinking of opening a Solo 401K and a Roth 401K. I want to convert the SEP into the Solo 401K. When I'm done I can reduce my taxable income with the Solo 401K and can have the Roth for tax free withdrawals at retirement. Are the contribution limits separate or combined for these 2 401K accounts?