In most two-slice toasters, it is not necessary to keep one slot empty for warming up bread. Modern toasters are designed to handle both slots being used simultaneously, and they should be able to warm up bread evenly in both slots.
Toasters are equipped with heating elements on both sides of each slot to ensure even toasting. When you put bread in both slots, the toaster should apply heat to each slice independently, resulting in consistent toasting for both pieces.
However, there are a couple of reasons why some people might choose to keep one slot empty:
Faster Toasting: If you're in a hurry and only need to toast one slice of bread, you can save time by using just one slot. The toaster will heat up faster, and you won't have to wait for the second slot to complete toasting.
Energy Efficiency: When toasting only one slice, using just one slot instead of both can be marginally more energy-efficient. The toaster will consume less power since it's only heating one slot instead of two.
In summary, while it is not necessary to keep one slot empty, you can choose to do so for convenience or energy-saving purposes. However, using both slots in a modern toaster should not negatively impact the toasting process or the quality of the results.