Download Zipper's mirrors pack, or make your new with my Plane model
.
1. Load your XNAlara, and select Models.


2 make pose for model.


3. load mirror model, setup location/size.


4. click on Lara (model) and make her clone via File> Clone selected model.
5.click on Commands> mirror model (z for this situation)

THIS IS ALL!!!!

about mirror commands:

click on [mirror model X] we get left/right mirror pose relatively Lara
click on [mirror model Y] we get top/bottom mirror pose relatively Lara
click on [mirror model Z] we get front/back mirror pose relatively Lara