My last contract I was running an IT project for a consulting firm. Half the team here, half in India.
90% of the team in India had secret second jobs - it was about 40% here in the US. Most miserable team I have been associated with. The rest of the team shouldered the weight of the project, while the other didn't do much and complained about the workload. This wasn't a case of not having a lot of work to do and plenty of time for a second job. Each of these jobs should have been a normal 50-60 hour a week commitment.
I have had two jobs in my career that I could have made it happen - but they were odd circumstances for me. For those jobs - go for it. Sounds like that's what was happening at the beginning of this thread - so no shade that way. There are also those that try and undershoot the jobs - get two jr developer jobs instead of 1 senior developer jobs. Or just sign up for 3 jobs and be ok with people wanting you fired after 6 months because you'll have 6 months of extra income.
I've never signed an employment contract that didn't have words about disclosing outside jobs. I've never signed as a contractor that didn't have something in it that said the same - but my kind of freelance is always assumed full time. They absolutely can fire you for having multiple jobs if it goes against the contract - but you really have to be obvious.
I've never fired more people than that last project. If they really just couldn't get work done, we let them go. If we fought them on suspicious behavior and no work done we let them go. I heard other meetings going on during our 'project all-hands' with voices I didn't know. People fought coming on site even though it was a project requirement. People not available through the day on Teams - even if their Teams showed green.
There were two who were caught red handed. One said he was going on vacation in Hawaii and then the owner of the company ran into him at an industry conference with another companies badge. The other was stupid enough to work for a consulting competitor and ended up on a shared client. VP was so mad he was going to sue to get all the money back. It was bad because both consulting firms were submitting billing for 40 hours a week to different clients.