Huh ??? Why so difficult with 2 or 4 scales?
Just place the cat on the trailer and lift one hull up somewhere in the middle. then you shift a box or something else with the weighting scale on it under the hull.
Do the same on the other side (on the same place of the other hull!
Add and done!
accuracy = 2 x scale accuracy + 2 x reading accuracy (only with analog scale)
This method is likely to produce errors. It is essentially a three point lift with one point on the scale. Then doing a mirror image on the other side.
If your trailer is symmetric and you place the scale under exactly the same point of the hulls, I don't see any inaccuracy.
Even if you miss some centimeters or having it not exactly balanced on the same level, the theoretical fault you make, is neglectable and falls problably within the inaccuracy of the scales
If you question the accuracy of a measurement, it's very easy to test this:
roll the trailer 1 meter further and start the measurement again starting with the last hull.
Check the difference with the first measurement. If it's too big, then I eat my shoe
