Can I wire two 12v atx connector together to get 24v? or the 12v and the 5v for 17v? I have a small amp board that I want to power with it. It's technically possible, but a really bad idea if you ...

I recently came accross a supply of 24V Panaflo 80mm "L" fans. The Panaflo spec sheet shows these running at the same rpm and noise level as the 12V variety. Can I safely undervolt this fan and run it ...