Ya'know.. I'm not really sure how much axles flex. I would imagine it would be weight dependent. They are made with a slight "bow" so that when the weight of the rig is applied it will zero the camber on your tires.
Every time you hit a bump in the road your tires can go "light" as the weight rebounds and the axle itself will "flex". That would be compounded by welding attachments between two axles. You would get independent flexing between the two as well as a bit of twisting action.
I'm not a welder, but I would think that would be rough on a rack like you were proposing.
I've been wrong before...