Good question. It would certainly take more energy to cook two pies (say) than one pie. However, if your oven leaked energy copiously (e.g. a badly-fitting door) then you'd probably do well to cook two pies, not one, because so much of your energy is wasted anyway.
It's always tempting to run an experiment. Try taking an electric meter reading before and after cooking one pie, then do the same after cooking two pies. You'll have to make sure no one ruins the readings by having a cuppa while your pies are cooking - boiling a kettle takes a lot of energy. If you have a gas oven then all bets are off.
