It's really cool how such a simple idea (taking the average of two facial expressions to obtain a facial expression in between them) is used so often in computer animation!
Are there bounds on the delta x and delta y?
Great! Re:: bounds, when you want to learn the blendshapes, it is likely a good idea to regularize these so that the magnitude of the offset won't be too large. In practice for animation often artists draw this so it's up to them :)