Is this a Bug ? I'm not sure.
Having some Gunzo dogfight with the Su-25, I was able to use the Aiming pipper from CAC mode [6] and the Fixed reticle sight [8] at the same time ?!
I thought the [8] reticle was just a backup mode that replaces any other HUD indication.
EDIT : True for any combat mode, Ground [7] or CAC [6], with the Su-25 only ; no way to have both on Su-25T or any other Russian fighter.