We need to know how many channels your amp is, if it's bridgeable, how many voice coils are on your subs and what the ratings are on your amps and sub and at what ohms.
If you have 2 subs each with a single 6 ohm voice coil, you will be able to get to either a 12 or a 3 ohm rating on a single channel amp. If you have 2 channels you will also be able to get 6 ohms. What that basically means is that is how much resistance your amp has to go through to power the subs. The higher the number the greater resistance.
Your amp has ratings at different ohms. Most amps are stable as low as 2 ohms, while the competition ones can go down to 1 or even lower. An amp that is rated for say 400 watts at 4 ohms should be rated at 800 watts at 2 ohms.
Here is a link to some diagrams that may help you understand it.
http://www.the12volt.com/caraudio/woofer_configurations.asp?Q=1&I=62