If you're two-timing on clients / employers (that is, plainly working two FT jobs at once) -- you can't expect that to end well. If you're working > 50 hours a week (or whatever your personal limit is) for more than 1-2 months -- that won't end well for your health or business relationships, either. That's the score, basically.
My F/T job at a FAANG adds up to about 10 hrs/week at most.. sometimes 15 hrs due to extra meetings. Could easily fit in another job, but would rather sit in the sun and read a book.