if I connect a 12 v to positive and 12- to negative is the voltage 12 or 24? Does it affect the current?

 

Voltages are measured relative to ground. There is a supply that provides 12 volts above ground and there is a supply the provides 12 volts below ground. It is not considered a 24 volt supply.

 

If you have a common pin then that is 24V, the 12V can be provide more current as 25A for a 350W psu, and 0.8A for -12V, what is your purpose(application) for this psu, you can see that the current is unbalance, show your psu that it has a label beside the psu and you can see all the details of voltage and current.

 

The -12V side of a computer PSU cannot usually sink as much current as the +12V can source. For example the +12V side might be rated to 1-2A while the -12V side is rated to just 100-200mA. The voltage between them is 24V, although it may not measure that way without a low-pass filter. But either way the current is limited by the weaker link.