Situate the two halves so that they just BARELY touch, then attach one to the other making them one object, then switch views to see the vertices. Go to the Weld panel and set the radius to like 0.01 and select all the verts on the model. Click SELECTED and it will weld all the verts that are really close together (ie: your two halves where they barely touch).
EDIT:
Whew! my spelling STUNK!