First off you have two methods of applying Plasma
- Projectile weaponary (Torpedo/Mines)
- Energy weaponary (Beams/Cannons/Turrets)
Now it is only ever possible outside of Officer abilities (which none are available) to place only two stacks of Plasma debuffs.
One coming from Projectile Weaponary and the other Energy Weaponary.
But I've seen Plasma debuffs on my target (or me) stack up twice)
Plasma debuffs can stack only one per player. When there are 2 players firing Energy plasma weaponary the stack will increase to 2 if both procs exist within the same time margin. Same thing occurs for Projectile.