Does separate alpha blending in shaders work?

In the Unity documentation (http://docs.unity3d.com/Manual/SL-Blend.html), it says this about blending syntax:

Blend SrcFactor DstFactor: Configure & enable blending. The generated color is multiplied by the SrcFactor. The color already on screen is multiplied by DstFactor and the two are added together.

Blend SrcFactor DstFactor, SrcFactorA DstFactorA: Same as above, but use different factors for blending the alpha channel.

However, I can't get SrcFactorA and DstFactorA to work and produce separate alpha blending. I can't even get it to break on purpose using "Zero Zero" or any other combination... as near as I can tell, everything after the comma is flat-out ignored.

Does anyone have any examples of a functional shader that successfully uses separate alpha blending?