Multishot and padding Serpent String are different. In strictly DPS in a RBG as Survival currently with Tuanha's CR does more padding.
I guess I don't understand what you're asking for then. Both Multishot and Arcane place Serpent Sting on the target, and the routine is already designed to go through the typical DPS roation of Single target or AOE depending on X amount of targets, so Serpent is going to be up pretty much permanently. You can always set the number lower for Multi if you want more people to get Serpented in smaller groups.