shape keys made from other shape keys Is it possible to make new shape keys using varying amounts of other shape keys.

For example let's say I have two shape keys one for the upper lip and one for the lower lip. Can I create a third shape key using a bit of both previously created shape keys to move both lips at once to spare time?
It's possible and easy (check image below). By adjusting value of previous keyframes, you get different mixtures of them.