This paper describes a technique to do both transitional blending between animations and layering of two animations. Layering is when two animations is run simultaneously and the result should be the combination of them. Transitional blending is the transition from one animation to another, for example running to idle animation.

A picture showing layering:

And a link to the paper:
http://graphics.cs.lth.se/theses/reports/jeppsson_masterthesis.pdf