Yes the total weight is the same whether you measure one axle at a time and add them together or just put both axles on together for a total. The only weigh I could see it being different is if the ground leading up to or away from the scale was higher or lower than the scale itself. In this case I would imaging the weight could transfer onto or off of an individual axle differently based on which axle was higher or lower at the time of measurement.
All the scales I have been on have been flat so my individual axles always add to the same total.
One of my most recent checks was
Front 5050
All 12,400
Rear 7350
If you add the front weight to the rear weight it matches the all weight.