How to wire a 12V DPST switch?
That is an illuminated switch, not the simple DPST that you indicate in your question.

Since Vellman doesn't provide a datasheet, you will have to do some testing to find how the LED is connected.  You may find that one pair of terminals is for the LED, and the other is for an SPST switch.  If you ask Vellman, they may be able to supply more infromation on the switch, or may be able to direct you to the actual manufacturer for information.

A common recommendation here is "No datasheet, No Sale", as you often need information from a datasheet to properly use an item.