Would you cut a ten dollar bill in half and use as two five dollar bills? Of course not. It's silly to even suggest something like that. Likewise, cutting a postage stamp in half would invalidate it immediately and the stamp would cease to be legal. But there was time, not very long ago, when the post office used to honor bisected stamps.

Unless you are into philately, you might not have heard about or seen a bisected stamp. These are stamps that are cut mostly diagonally across and used in post to pay half its face value. For example, a ten cent stamp could be cut in half and used as two five cent stamps. Some went even further and cut stamps in thirds and other fractions.

A bisected stamp used in the Island of Guernsey during World War 2.