Well in Germany specifically it's a legal issue (and some other EU countries), because there are labour market laws stipulating the maximum amount of time someone can work per week (48hrs).

Sure, you can fly under the radar if you're working on your own business/start-up by simply lying about the hours worked, but if you're doing paid work for another employer it will be obvious, and they would not be legally able to employ you if total working time goes above 48hrs/week.

Then you are dreaming (because of course running a second demanding company will affect your work, in some way, not necessarily negatively) but, more importantly, people might disagree about the impact and either side would have a hard time proving anything. So probably be upfront about the rules and make a choice, when you have to.

That may work if you’re coding in you’re spare time on a side project that may turn into a business at some point. But even then, you’re going to be more tired and drained at your main job—humans just aren’t capable of working two jobs for a prolonged amount of time without one of them being negatively affected. Sure, you might be the one out of thousands that manages. But most are not.

And we’re not talking about creating a small online T-shirt store, but being CEO of a venture backed organisation to evolve the state of AI. There’s just no way to pull that off.