I don't know if hitting a new enemy resets the spears or no, like Vayne's silver bolts, or if they stack up like Twitch's passive. But imagine if they stacked up like Twitch's passive and you built a Runaan's. The fact that Rend resets on making a kill would be horrifying.
There's more to consider than that.
Is Rend a single target nuke or an AOE like Twitch? Can they stack on multiple opponents like Twitch passive or only on the main target like Vayne?
By the looks of it, rend may just be like "if you got a knife in the back" thing.
Think of Sejuani's passive. If you auto-attack or use an ability, they will get frost.
If you use E, then you deal damage to whoever is frosted. I'm pretty sure you can stab multiple enemies then rend them all at once. If not, then I'm sad.
