[GigaFrog]

This is probably because you don't load the 5V output of your PSUs.
To work properly, a AT or ATX PSU needs load on the 5V line.
When you run at 24V, both 12V outputs drop by 2V, leading to a total loss of 4V (20 instead of 24V).
When you run at 17V, you have (I guess thats what you do) a 12V output in series with the other PSU's 5V output. In this case, the other PSU will correctly regulate its 5V, but the first one, runing only its 12V, will still drop by 2V. Total loss = 2V in this case (15 instead of 17).

An advise when you start messing around with PSUs: buy a multimeter, it is cheaper than a PSU, and you will eventually end up spending the money once you will have fried a PSU. The multimeter will help you understand what is goind on, and prevent misusing your PSUs.

Have fun!