Offset Tween Curve

Hello beautiful helpful people 

I am trying to produce a triangulate truss, but I cannot for the life of me work out the correct offset (either planar of vector based) for my tween curve. 

I have got a base catenary which has been NU scaled (I would like to keep the vertical trusses vertical at the point they come together but with a slight gap (see right viewport)) 

I am wanting to offset the points tween curve proportionally to the distance between the the two catenaries at that point, (i.e. produce a line between the control point of the base curves and then offset the points of the tween curve by a multiple between 0<1 of that length, then cull and triangulate these points to produce a truss. However offsetting it simply in the xy plane is not my desired outcome and I want each point to have the correct vector. 

In essence I am wanting to get a vector 2pt between the control points of base curves and then find the normal to these vectors (I think/hope)

Any help would be much appreciated 

J-P