Answer:
Depends a little on the sort of capacitor and what you want to do.
For example, if you have a pair of polarised electrolytic caps, you can connect the two negs together and then simply connect two wires, one to each positive to make a non-polarised electrolytic capacitor.
It isn't ideal but can certainly get you out of a hole. Some people (depending on the application) also place a blocking diode over each cap so that each cap cannot be reverse charged.
So....if this is waht you mean - then "connect the two negs together and then use the two positives only to connect to the circuit. Capacity will be half of the value of the two caps used.
eg: a 100U capacitor back to back with another 100U capacitor will give you nominally 50U non polarised capacitor.