Baking is a complex interplay of chemistry and physics. In a conventional oven, if you bake the same size loaf or cake layer, it doesn't usually matter how many pieces or dishes you have in the oven (but allow air to circulate among them). To bake twice as big, you may need to bake longer at a lower temperature to avoid overbaking the outside before the middle gets done. No simple formula, however, exists to apply to these situations.
Custard Pie
The classic recipe on Libby's pumpkin cans (15- or 29-oz.) makes one or two 9-inch pies, and they bake in the same time, 40 to 50 minutes. If you make one deep-dish 10-inch pie and one 8-inch pie for leftovers, the shallower pie may simply finish on the low end of the time span and the deeper one on the high end.
Yeast Breads
When the chemistry of leavening is involved, the danger of too hard a crust is greatest and you're best advised to keep to the original-sized piece of dough, or at least its thickness in the middle.
Layer Cakes
Cake recipes generally have their leavening tuned closely to particular depths of batter. To make twice as high a cake, bake twice as many layers. They'll bake more evenly, and you get twice the filling in between. Check the labels on specialty cake pans before you buy for advice on adjusting baking times and temperatures.