Toaster ovens require more time to toast two pieces of bread compared to one because of the way heat is distributed inside the oven.
When you place a single slice of bread in a toaster oven, the heat from the oven's heating elements can directly and evenly surround the bread on all sides, toasting it more efficiently. The heat has fewer obstacles to reach the bread, so it can quickly penetrate and toast the slice.
However, when you put two slices of bread side by side in the toaster oven, they block some of the heat from reaching each other's sides. The heat must now travel through both slices of bread, making the toasting process slower and less efficient. The toasting time will be longer because the heat has to work through the first slice before reaching the second one.
Additionally, when you add more items to the toaster oven, it may cause the temperature inside the oven to drop slightly. The oven's heating elements will work to compensate for the loss of heat, but this can also contribute to the increased toasting time.
To ensure even toasting, you may want to flip the slices halfway through the toasting process if your toaster oven does not have a feature to rotate the food automatically. This way, both sides of the bread can receive a more even amount of heat for consistent toasting.