Hooking them together would provide 24VDC. On a 350W PSU, I see -12V rated to supply 0.3A, which would allow for 24V fans drawing up to 7.2W of power, if desirable.
I don't see any reason to prefer 24V fans, however. The current losses over such short ranges are minimal, and I would think the larger economies of scale of 12V devices will keep 12V fans less expensive.
This is false. ATX provides both +12V and -12V: * https://pinoutguide.com/Power/atxpower_pinout.shtml
Hooking them together would provide 24VDC. On a 350W PSU, I see -12V rated to supply 0.3A, which would allow for 24V fans drawing up to 7.2W of power, if desirable.
I don't see any reason to prefer 24V fans, however. The current losses over such short ranges are minimal, and I would think the larger economies of scale of 12V devices will keep 12V fans less expensive.