Help needed building an onager

[P. Clodius Secundus]

Wouldn't it be easier to make a single rope whinch ( I know 2 ropes are stronger) because this migght get a simetric and so get some torsion on the whinch??
btw i made a scale model for my daughters "sinterklaas surprise "to shoot "pepernoten"they came about
6 meters. but i works.

Agrimensor,
True, a single rope winch might be easier to build, but it would pull the arm out of alignment as it wound around the drum. Technicalities like this are seldom a problem at the force levels we generate with hobby projects or even larger scale models, but they can greatly impact the dynamics of full sized weapons.
From an historical perspective, Ammianus said "...four young stalwarts on each side, by pulling rearwards the bars to which the withdrawl ropes are connected, draw the arm down almost horizontal". Since he is our only real source, two ropes makes more sense.
I hope your daughter enjoys her present. I helped my friend's daughter build an egg trebuchet for her school project. It was fun and educational.