Defense Down effects such as Acid Bolts, Angon, Spirit Surged Jumps, Weaponskill effects, Blue Magic effects, etc do not stack. Only one may be active at a time.
Stronger effects sometimes overwrite weaker ones, such as the case with Angon
Stronger Defense Bonus effects such as Cocoon tend to have a higher priority than and either prevent or overwrite Defense Down effects, but not always.