What is wrong with my cars subwoofer system?

I have two pioneer 12s in a ported box both are 1000watts max and i have a massiv amp that runs 800watts rms its a mono amp so it has one output. i have had the system connected to both subs before and i has run fine with no problems at peak performance of 800 watts rms but once i took the subs and amp out only for a trip where i needed trunk space n when i put the subs back in i plug back in the second sub and they both power onto like a super low bass sound not like at their normal levels where they hit loud. When i unplug one of the subs the other will work perfectly fine at optimal performance. what could i have done wrong during hook up to have them both not work? what could i do to make them both work at peak levels? in my ported box which i had them in the entire time.