I see a lot of people talking about reputational risk around this, but I'd be more worried about the legal implications. Most of us have contracts that grant IP of what we create to our employer at least during work hours - if you get caught doing this how do the IP implications unwind given that both your employers have the same rights to what you produced? Would it be legally equivalent to selling a bunch of IP tha…
That's an American thing. I work in Central European country, and IP that I create belongs to me until I pass the rights to my employer. Conveniently, my contract states that the act of committing my code to employer-hosted git repo is the act of giving up the copyright.
You can easily do that for two employers at the same time.