Additional Time Needed for Baking Two Loaves of Bread at same time?
Unless your oven is dramatically under powered, you should not need to adjust the bake time at all&mdash;and if it is so under powered, maybe because it is a counter top oven, you should only bake one loaf at a time.

Put the loaves in the oven with at least a hand's width of distance or so, allowing the air to circulate between the loaves.

Note that bread should be cooked until it is done, not just to a certain amount of time.   Your recipe should provide a doneness test such as a target internal temperature (preferably), or how to gauge by the color of the crust development.