My last startup failed because of this very thing. Except that I was the tech person and he was the salesperson. He just did not understand the tech so he could sell it well. He was a highly successful salesperson but he just did not get the tech--so the company failed. I know many here are super positive about these issues and indeed a properly setup instrument can protect you from a usurping co-founder but if you do not understand the tech well things can go wrong. I suggest you get a tech adviser like myself to check what the person is doing (of course also get a co-founder agreement) and if all is going well. Set benchmarks that the tech person must meet in an agreement to keep him or her on their toes, Keep control but you can only keep control if you understand what the tech does and how it will help the world.