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.
Theoretically, it could work if everything is absolutely level and there are no torquing forces between the beams and hulls.
Fat chance of getting all those variables out.
Like Matt says, two scales will do it. All of the weight on one or the other scale at the same time. Calibrate beforehand with a known weight close to what you will be weighing.
Mast up is OK if there is no windage. Even then, if you read both scales at the same instant you will be fine.