I'm not sure that I get it:
"Use this drive exclusively to keep a copy of every file that is targeted for duplication"
We have that. It's called the Drive Usage Limited balancer.
If you simply select that only 2 drive can store duplicated files on a pool, the you're effectively saying the same exact thing. Use this one specific disk to keep the original file and this other specific disk to keep the backup duplicate, and you can do this for 3 disks as well.
The thing is, there is no difference between what you call a backup duplicate and what you call the original. They are identical in every single way. You can choose to call the first one the original or the second one the original, or just call them 2 identical copies, it makes no difference at all. It's simply semantics.
If you want to do the exact same thing on a folder level, you can now do that too with File Placement in StableBit DrivePool 2.1+.
Maybe I'm missing the point here?