Yes, it works like that. Assuming you don't have a fixed version of the mod that prevents their use with anvils.
Well, it works almost like that. Rather, the total number of souls from both shards are added together, then a bonus is added on top of THAT, so odds are what you'll actually end up with is a tier 5 enderman shard. The resulting type is whatever you put in the left slot, and you get to preview the result before you collect it from the anvil.
Though it beats me what you'd do with one of those. Due to their ability to teleport, you really want to isolate such a spawner as much as possible - which is easiest to do in the End, where the enderman spawn rates are through the roof anyway (move away from the main island a bit and it becomes very easy to control their spawning regardless as to whether you're using soul shards at all).