The answer is...it depends!
If the SEP IRA and Solo 401k are both connected to the same business, then the contribution limits max out across plans at $57,000 per participant (or $63,500 if you are age 50 or older and able to take catch-up contributions).
That's because retirement plan limits are calculated based on the business connected to the plan.
Solo 401ks have two types of contributions: Employee and Employer contributions.
SEP IRAs, on the other hand, only allow for employer contributions.
According to the IRS, the employee contribution limits to a 401k are per person, where the employer contribution limits (including a SEP IRA for the self-employed) are per plan.
In this example, both the SEP IRA and the Solo 401k are connected to the same business, so the maximum contribution limits apply across plans.
However, if your SEP IRA is for one business and your Solo 401k is for a completely separate business, you may be able to maximize even greater contributions.
Check with your CPA to see if controlled group rules apply and may limit your contributions.