I know a guy that left one FAANG recently which was laying off people, interviewed at two other FAANGs, got offers for senior remote roles for over $600k, accepted both and is now working two jobs at the same time. Given he was doing the previous FAANG together with a tough grad school, I think he'll be fine.
I'm not in the USA so I'm not sure how the W2s and SSNs are tied to job/healthcare/401k etc, but how does this double timing work? Won't the Tax office/Finanzamt (IRS) know about this?
They can also request that the employer withhold additional taxes; this is fairly common because of situations where both spouses work.
They likely have two healthcare plans, but that's generally not an issue - again, it's fairly common if both spouses are employed.
For the 401K, the individual will just have to make sure that their total contributions between both companies are under the federal limit.