The roof of a small two-engine plane was sheared off by the wing of a TWA MD-80 jet in St. Louis last night, killing the two passengers on the tiny propeller aircraft as the commercial plane was about to take off. Eight of the 132 TWA passengers were injured. The commercial jet had been cleared for take-off and was taxiing at a speed of 80-100 m.p.h. when the pilot saw the other aircraft and veered to the left to try to avoid impact. Airport officials are still unsure why the smaller plane was there; it should have been waiting for clearance on a smaller, parallel runway. The Denver-bound jetliner was evacuated, and the passengers were placed on later flights during the busiest travel period of the year.