Subs in my car wont work?

I have 2 Dual SD12 subs hooked up to my Dual XPE 400 watt amp. I dont really know much about this stuff but my friend got it for me and helped me install it. the amp turns on but the subs dont do anything. everything is wired correctly, ground, battery, REM, RCA, etc. if theres anything anyone can tell me it would be greatly appreciated. also the subs are both 4 ohms, the amp is 2 ohms, 2 channel amp. i dont know if that helps.