This money.SE question and this BogleHeads forum thread both seem to ask about whether or not you can contribute to both a SEP-IRA and a Solo 401(k) at the same time for the same business. I'm not getting a clear answer anywhere.
If it matters, this is in regards to an LLC filing under Subchapter S. I'm hoping I can do a SEP-IRA now, and set up a Solo 401(k) if I hit the SEP-IRA limit. Can I do this?