You would want to add two primitives to the same collisionskin.
Inside your object you can just add another primitive like this.
body = new Body();
collision = new CollisionSkin(body);
//First primitive your handle
collision.AddPrimitive(new Box(position1, orientation1, sideLengths1), new MaterialProperties(0.8f, 0.8f, 0.7f));
//Second primitive your blade.
collision.AddPrimitive(new Box(position2, orientation2, sideLengths2), new MaterialProperties(0.8f, 0.8f, 0.7f));
body.CollisionSkin = this.collision;
Vector3 com = SetMass(1.0f);
collision.ApplyLocalTransform(new Transform(-com, Matrix.Identity));
this.scale = sideLengths;
You can add several primitives to one collisionskin.
For something as simple as a sword though i'd suggest using only one box unless you require really precise collisions.
Hope this helps.
Btw the closest thing to a "model object" in JigLibX is a TriangleMesh but there for Immovable objects only(like rocks, houses, etc)