Parameters for this alpha blend

I was asking about this at the end of another thread and I think it is not read any more, so I will try in a thread which is about this subject and not something else.

I want to get a blended alpha value which works like this

if two surfaces are drawn on the same pixel and one has alpha=0.75 and the other has alpha=0.5 then i need the result to be 0.875.

If the two alphas were 1 and 0.5, i would need it to be 1.

I can see how this is done by 1 - (1-a1)*(1-a2)

What I can not see is how to set the alpha blend arguments to get this. There is no multiplication operation, and the operators which exist, doesnt seem to do what i want.

The point of my blend is to get a resulting alpha value which defines how much of the background will show through the graphics.

Can anyone provide some insight into how to do this

Edit:

I might add that the colors should be alpha blended as usual. This is for the seperate blending of the alpha values.