This isn't a problem, but hiring people you don't trust is a problem. Part of that is setting expectations with a contract. Most of that is accepting that it's going to be a risk, no matter what you do (there's no DRM that will protect your code from your own engineers).
I recommend you don't worry too much about this in particular. Have some basic legal protection with a contract, that should prevent another company from forming. Someone running off with your code and dominating your company without a legal entity of their own... would be unexpected.
Also, your code is unlikely to be so particularly novel that the specific implementation is your secret sauce. (Not trying to insult you -- maybe I'm wrong -- but generally the idea of the source as a trade secret, unless we're talking Carmak's inverse sqrt, it's not that big a deal)