If two people always agree, one of them is redundant.

There are always two people in every picture: the photographer and the viewer.

If two people were exactly alike, one of them would be unnecessary.

One room is always enough for one person. Two rooms is not enough for two people. That is one of the conundrums in life.

There are two types of people in the world, and I'm one of them.

A suicide kills two people, Maggie, that's what it's for!