The “here is one hand” argument by GE Moore against skepticism against the external world says: here is one hand, here is another hand; there are at least two objects in the external world, and therefore the external world exists.
But this makes the mistake of assuming that idealists do not understand that duality is valid within the dualistic world. For example, within a game, you can show two weapons, but outside of the game, they are merely electrical signals.
Likewise, idealism is dealing with describing the ultimate reality, and how the dualistic reality originates from the non-dual reality.