My old ATX PSU has -12V so presumably one would get 24V across the -12 and +12?
You can only connect PSUs in series if there is no connection between the PSU output and mains ground. Some common power supplies (e.g. ATX PSUs and laptop PSUs) do have a connection between the 0V output connection and mains ground.
Yay! Another reason to get a "proper" Lab power supply instead of hacking a computer ATX power supply.