If the amp can handle the low impedance: run the positive wire from the sub you have to the positive terminal of the new sub, and then do the same for the new sub's negative wire to old sub negative wire. When you run two subs like this you are running them in parallel. This cuts the impedance in half (two 4ohm subs in parallel equals the impedance of 1 2ohm sub) which will make the amp create about twice the normal power.
