STAMFORD-City fire investigators say that a blaze that scorched one home and destroyed another late Monday afternoon while displacing 32 residents was ignited by an electric overload in a tiny illegal dwelling unit. Chief Fire Marshal Charles Spaulding said his investigators as well as state fire marshals found a five-toot by seven-foot illegal dwelling on the first floor rear porch of 50 Alden Street, where the fire began. In the tiny room made of plywood and two by fours was a mattress, microwave, air conditioner, space heater and other appliances plugged into power strips that were led to overburdened electrical outlets by extension cords.