Armor mashup (more than one .nif file) problems

[AndyIE]

I'm having some troubles creating an armor mashup for new vegas, I've loaded a nif mesh and imported another. while everything seems fine in nifskope/outfit studio. The imported nif remains invisible in-game and the geck. I'm completely new to this and having trouble finding help. Any information would be appreciated and thank you.

[AndyIE]

I've gotten the meshes to show up in the geck. But when applied to an npc and click preview, it crashes the geck...