Hi Paul,
If you have got known to be accurate (tested) bath scales you will be able to weight one wheel at time. The procedure for accuracy is to set something like a block of wood for resting the un-weighed wheel to the exact height of the other wheel being weighed on the scale surface.
IE; set the leveling block apart at the wheel base center distance, pack it to height using a straight edge and a level. Add the two results together, and it will be as accurate as your scales.
I am creating a reference on all that I build. When I’m posting stuff for eBay? I take test objects to the post office and write on them the result. I select them to calibrate my scales when I am operating close to that range (zone).