The way minecraft currently works is that fences can be placed on top of one another (indirectly), but to do so you need to build them from the top down (because fences aren't destroyed when the block under them is removed, and then a fence can be placed under the previous one). What my server member have expressed interest in is a mod that would allow them to build from the bottom up, as they would with practically everything else. I'm not sure if this is even possible, but I figured I'd suggest it for the few users who would like it.
