Got a pic of what you're trying to accomplish? You said you could merge them in nifskope but the collision is wrong?
I managed to wrap an ear around Blender well enough to be able to cut off vertices to fix collision problems. Sounds like that's probably all this would need is some vertex adjustments.
I don't have a pic (maybe I can make one tomorrow) but basically what I'm shooting for is an example of the typical CastleIntNarrow or Basemant wall that has two sides so that you could use them as room dividers. This is in cases where the floor and ceiling on a typical wall won't do for whatever reason. In one instance, for example, I need to divide the top half of a barracks tower into two rooms like the bottom floor. In another, I want to place a divider in the middle of the room like a partition (with both sides free to have a table next to it). It doesn't need the pillars or posts on either side, however - just the walls and the trim along the bottom and/or top. What I was trying in Nifskope is stripping out the ceiling and floor from a 2wayroom mesh and putting two of the 1-way walls back-to-back, but that way the collision is still there for the ceiling and floor but not the other side of the wall. Thanks a bunch if you do this for me!