Attack Down effects such as Demon Arrows, Weaponskill effects, Blue Magic effects, Ninjutsu, etc do not stack. Only one may be active at a time.
Stronger Attack Down effects sometimes overwrite (and prevent) weaker ones. Such as the case of Fatso Fargann's Acid Mist (-50%) with Generous Arthur's Corrosive Ooze (-33%).
Stronger Attack Boost effects tend to have a higher priority than and either prevent or overwrite Attack Down effects, but not always.