Thanks for your replies.
Clootie - Spasibo

But I'm still stuck.

Try clearing render-texture with $00FFFFFF - A=0, R,G,B=$FF. And, of course, use norma srcalpha/invsrcalpha combination.
Tried that. It had some effect in certain blend setings, but no desired result

If it doesn't help, try examining alpha channel of rendered render-texture -> use D3DTA_ALPHAREPLICATE.
If it is not too much trouble, could you provide a link to an example. I couldn't find anything that made sense to me.

One of the ways:
(1) set S=ONE D=ZERO / render to rendertarget surface. - now RT contains the copy of your texture.
(2) render to backbuffer as in your first alg, but using newly genereated RT texture.
This actually worked , but only if you render one sprite from texture to a rendertarget surface . If you render two overlaping sprites it doesn't work.

I tired to figger out how to give you more info, on what I'm trying to do. May be this will help:

If you ever worked with photoshop, you know it is all about layers. So if you follow the below steps, you should get a visual picture of my problem.

1. Create a blank photoshop RGB image with white background
2. Create first layer and draw a blue circle on it
3. Change opacity of this layer to 75%
4. Create another layer and draw a red circle on it, which overlaps partialy with the blue circle on the first layer
5. Change opacity of the second layer (with the red circle) to 50%
6. Merge second layer with the first one (Layer->merge down)

now we end up with a white, non transparent background and one layer, with completely transparent area around circles, Part of blue circle (75% opacity), part of red circle (50% opacity) and purple area, where two circles intersect (?% opacity)

This is what I want to achieve in d3d. White background is out backbuffer, Layer with two circles - our resulting rendertarget surface after sprites are rendered on it.

I tried all sorts of combinations of blend flags. Non of them give me this result.

Also, in this case, what would be the formula to calculate opacity for that purple intersection area. It is not completely opaque, but it is clearly more than 75%.

I'm almost ready to give up. I've wasted two weekends on this. Apart from this crap, my sprite rendering engine is complete.