This is the cause of your "undithered" looking capture. Asuming pixelsize is nearly identical on both cams you dithered your main cam 2.8*1000/1200=2.33 pixel.
This is near nothing. Hot pixel or "ununiform behaving" ones are neighboring themselfs in the stack, this makes the rain fall.
Dither at least 10-20Pix with this configuration to separate them for sure.