Add effect won't proc if you have runes up and the same effect won't stack from multiple swords.
Lunge is only a single attack every 3 mins. You obviously want to get the most out of it, but it's not something to build around. If you gambit > put runes up > lunge put runes up, you are already eating into your overall DPS unless you have very little haste. Gambit > lunge is good for helping to remove auras though.
Personally I only lunge when runes are due to start dropping or if i want to remove an aura, unless I'm fighting fodder (read: Umbrils).