There are. The Kinesis keyboard has four thumb buttons on each side of the keyboard. The biggest, closest one on the left side I map to CONTROL. The other big left thumb button I map to ESCAPE.
They have two thumbs, but within reach of their thumbs are extra keys. They need these extra keys because there are a lot of characters in their writing. They don't need more thumbs, because they only need to push one of these extra keys at a time.
Do I need more pointer fingers if I need to use one of the Y, H, N, B, T, G, and V keys?