How to blend multiple alpha channel WMVs with WPF MediaElements?

I have two mediaelements overlapping each other, on top of a static image. The media elements are each playing a different WMV file each with an alpha channel defined.

(top)

A - WMV video 1 with alpha channel

B - WMV video 2 with alpha channel

C - static image

I have two questions:

1. How do I get layer B to bland with level C and level A to blend with B and C so that levels of transparencies are calculated and blended together through the three levels?

2. Should I be using a different video format that would be easier/snappier/lossless rather than WMV with my mediaelements开发者_如何转开发?