two particlesystems - visibility problem

Hello,

I'm using two particle systems. One particlesystem (a) is in front of the other particlesystem (b). But particles of (b) are rendered in front of particles of (a). I tried the following adjustments:

1. changed the sorting fudge of (a) to -1; changed the sorting fudge of (b) to 1

2. tried all available particle shaders

3. added ZTest Gequal to the particle shader of (b) => nothing from (b) is rendered

4. perform a raycast from particles of (b) towards the direction of the camera -> lifetime=0 if ray hits something -> but it seems particles don't have a collider?

I should mention that there are some gaps in (a), therefore some particles of (b) should appear on the screen. But not all! :-( Nothing of the approaches above works. Sadly I have no ideas anymore... maybe somebody can help me :-)

Thanks in advance!