endiz:
You should consider why you even want this though. If you get to a place where they are even, then most free is going to write one, then maybe write the other, then maybe write the first, etc., simply because each disk briefly has the most free. High-water is the default because it is a compromise between distributing the files, and constantly switching disks.
Unless your first disk is really nearly full you might just let it alone. Rather than worrying about even distribution, you should make sure you set the Minimum Free for each user share. Minimum Free should be set larger than the largest file you expect to write. It has no way to know how large a write will become, so if it has more than Minimum Free, it can choose the disk and if the file is too large, it will fail. If a disk has less than Minimum Free, it will choose a different disk.