Move one side so both are not exactly in the same place. When two or more thing meet and they seem to share an edge, the best is to grow or push them so they just overlap by a tiny margin. The printed result will be similar to what you want, but the computer processing the model will have a enough info to solve the zone.
Where the blades cross, they are creating two inside edges. It looks like you're using SketchUp. First, select the entire sword and right click > Intersect Faces > with selection. This will make certain that all the faces in the model are joined.
Now select the face of a blade and right click > hide. You should now see the center where the two blades meet. Delete this until you see the other side of the second blade. Repeat this over the entire model until it is literally hollowed out.
This might help you a bit: Linkyhttp://fracturedmesh.blogspot.com/ - my 3d modeling blog