What was found
AIRCRAFT WAS GROUNDED: WHILE IN CRUISE AT FL370 THE RIGHT PACK LIGHT FOLLOWED BY THE LEFT PACK LIGHT ILLUMINATED. RAN PACK QRH CHECKLIST AND BOTH PACKLIGHTS EXTINGUISHED. FOLLOWED BY BOTH CABIN ALTITUDE LIGHTS ILLUMINATING. RAN QRC AND QRH FOR CABIN ALTITUDE WARNING, AND CONDUCTED AN EMERGENCY DESCENT. DECESION MADE TO RETURN TO MIA EMERGENCY DECLARED AND LANDED OVERWEIGHT WITHOUT INCIDENT. MAINTENANCE TROUBLE SHOT PER FIM 21-51-00, FOUND LH AND RH PACK WAS IN FLIGHT COMMANDED OFF, MOC ALSO DETERMINED BOTH PACK WERE IN FLIGHT SELECTED OFF, AIRCRAFT RESPONDED ACCORDANLY. ACCOMPLISHED LH AND RH PACKS IBIT OPS CHECK IAW AMM 21-00-00 OPS CHECK NORMAL.