My primary flight instructor said twins have two engines because they are too heavy to fly on one...
Really, aircraft design is governed by engine availability. They design the airplane around what can get it off the ground. The L-1011 was dead and bankrupting Lockheed in its final stages of development - until RR was rescued from bankruptcy.
Second design requirement is where to put the landing gear. If you don't want to put it in the fuselage like a BAe-146, then you gotta' make it a low wing, which limits the size of wing-mounted engines. See above.
"An airplane is a whole bunch of compromises flying in close formation."
GB