Can I have a Solo 401(k) and a 401(k) at my W-2 job at the same time?

Yes, you can contribute to both if you have a W-2 job with a 401(k) and a side business, but you have to respect how the limits combine. The employee deferral limit — $23,500 in 2025 ($31,000 if 50+) — is a per-person limit across all 401(k) plans, so your deferrals into the workplace plan and the Solo 401(k) together can't exceed it. If your W-2 employer match already uses up your deferral, you can still make the employer profit-sharing contribution to your Solo 401(k) based on your self-employment income, since the overall $70,000 per-plan limit is separate for each unrelated employer. In practice this lets a moonlighter capture their day-job match and still shelter side income. Coordinate carefully to avoid over-deferring, which triggers penalties. A common move is maxing the workplace match, then loading the Solo 401(k)'s employer side.