How to see particles through a fade material?

If they are both transparent, this is an “unsolvable” sorting problem in Unity's current rendering technology (and in most real-time rendering technology)

This is because the particles are both in front of and behind different parts of the glass. Research OIT (order independent transparency) or Depth Peeling for more background/detail. There is no currently widely accepted solution to this problem though - it is considered an open problem in real-time rendering. All known solutions have some drawbacks/limitations.

If the viewing angle is fixed, you could break the glass into 2 pieces. If the viewing angle is not fixed, breaking the glass into 3 or 4 pieces ought to be enough, but obviously increases your GameObject count and adds a maintenance overhead.

The only other way is to make one of the objects solid or cutout.