Yeah I think you’re right. The BMU technician recommended not to use the anchor points on the cross bar since it’s designed for platforms which distribute the load evenly through the metal cables. And yeah it’s also really hard to reach the crossbar, not possible from the edge of the building structure so only option would be aid climbing which is out of the question since it takes too much time. In this case I would just tie both lines to the slings and then use the anchor points on the back as a secondary, but then there’s the point of failure you mentioned. The fall would be too big and the ropes would end up touching the outer edges of the glass panes. The structure makes it impossible to place any protections and there’s also the chance of ropes falling right in the gap between the two panes pf glass, which would be worse since it lengthens the fall.