The weight seen at each individual tire can be added to get overall weight. Just stick a scale under each tire, add up all four numbers, and that's how much your bike weighs.
If you want to be exact, use 3 blocks of wood that are the approximate thickness of the compressed scale to remove the additional weight caused by the increased height of the wheel on the scale. Or, put the scale in a dug out hole that makes the scale ground level, lol.
