No, that's actually only partially true. First off, there's 'standby power', there's a certain baseline that has to do with firing up the switcher and it's fan. The bigger the supply, the more that power is. It's typically a few percent, a 400 Watt supply might draw 10 just waiting for something to do.
And there's efficiency. The supply is designed to be efficient where it really counts, at or near full power. It's not uncommon to find 50% efficiency at very low powers (relatively).
Collectively, this could mean your 400 Watt supply might need say 30 Watts to deliver the first ten. A better sized supply is probably closer to 80%, maybe more. Say 12 Watts?
This is actually not true, either. Have you tried it? I have.
What happens is the plus five supply is a current source (traditional current, plus to minus, if you're an electron flow guy think the other way.......funny though, it's still called a current source....). It cannot sink the current supplied by the load from plus 12 (again, hold it up to the mirror if you're left handed.....). No circuit exists.
To make it work, you need to apply a static load to plus 5 larger than what you intend to dump from plus 12. In the application I used this was just a power resistor, from this standing load current is automatically subtracted.
That's the way electrons work. Really.
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