How many amps would i have between the +12v and -12+ rails on a computer PSU?

I'm aware that i can obtain 24 volts between the -12v and the +12v on a computer power supply but i'm not sure about how many amps would it be able handle between the two.
Imagine that the power supply label said 20A for the 12v+ rail and 0.8 for the -12v rail. Would i have 20,8 amps? Or the least of the two, 0.8A?

Thanks in advance.

0.8A

As above - the lesser of the two rails.

Thank you!