Here is a patch in which I tried to illustrate the problem as clearly as possible.
Plane A intersects plane B, A and B are both half-transparent. But they never blend as expected, like in the attached image. Either A blends with B or B blends with A, depending on which one is rendered first.
I know this problem is impossible to solve due to the hardware restrictions of the graphics card. But there have been miracles in this community before, so in case anyone comes up with a trick, feel free to let me know ;)