This seems like it should be simples.
I have two quads.
I have a smooth fade on the edges of each one, applied to the alpha channel.
I then overlap the quads on this region and alpha blend.
How do I get the result of the blend to be the same regardless of which quad is drawn first? If it was in a shader it would be a case of ((X.alpha*X.colour ) + (Y.alpha+Y.colour))