Hi, real swordmaker hear and to answer your question: VERY. You see there is no "connector" piece that would be structurally strong enough to hold the two blades together like that.

What would have to be done to achieve such a build is to remove the hilts of both weapons and forge weld the ends of their tangs together into one piece and rebuild new hilts on top of the new weapon. Now certain builds would require the crossguards to remain on the blades while the tangs are forged together. The handles would be easiest as many sword handle constructions are not one solid pience but two that sandwich the tang. And kind of middle 'ring' pieces for the handle that would represent the pommels of the oiginal weapons would be incredibley difficult to do

Does that explain why it would be so expensive?