That seems unlikely to me, but maybe not to you? Or you're saying it's like plausible deniability, like nobody can prove they weren't, or something?
I wonder if GP who mentioned giving his two weeks notice on first day of new job... was planning on working 80 hour weeks for two weeks while they overlapped?
I mean, there were times in my career where I was working 12+ hour days for one employer, so working 16+ hour days for two doesn't seem like that much of a stretch; still leaves you with 8 hours of sleep. I've known enough workaholics in my time to know that it ain't beyond the realm of possibility.
But yes, it's more of a plausible deniability thing. Either/both employers would need to prove that you weren't working 40 hours a week for them, which is not only impossible (good luck proving a negative) but a much higher standard than is reasonable in the first place (especially for programmers, having a full 40 productive hours a week is rare and unsustainable; half that or less is more typical).
Because there are 168 hours in a week and each full-time job only consumes 40 of them.
Now, if the employers are competitors of one another, or there's otherwise something that would produce a conflict of interest, then that's another story - but beyond that? As long as you're fulfilling your duties for both jobs, it's neither employers' business what you do on your "free time".