Your insurance companies are likely to disown you if both are offering design services. Other than that, it's really up to the two companies and what their policies are. In terms of being listed on a two companies Certificates of Authority, I don't know of any restrictions, but I doubt there would be anything stopping you legally. Lots of people own and operate multiple businesses.

Why? the only common reasons I can think of are for unethical reasons. Either that says something about my mind or the intention :)

Definitely a question for your insurance company. I think they are okay for you working for one, two, maybe three companies, but they draw the line pretty early on.
The question from me is...why??? Put your focus (Principal) on one company and take a more advisory role in the other if you are really hot-to-trot about having multiple irons in the fire.
If the companies are doing exclusively different activities, then it may be less of an issue.

It's done all the time. Contract engineers often work for more than one entity.

Who is employing you? Not much detail to go on here, but if you essentially work for yourself why not? You should know yourself if there is any conflict of interest, but as you don't give us much we can't give much back....

If you work for one company it depends what their terms of employment say.

Or is the term "principle engineer" specific to your region?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Yes you can be the principal engineer for more than one company. Most of the challenges are related to conflicts of interest and competition for tendering.

If I remember right, some states require the qualifying engineer in an office offering engineering to be a full-time employee, which means you can't very well qualify two different companies yourself, and that would be one limitation.