Just based on the understanding that a moment is to go through the two welded plate joints, a possible approach to indicate the loading seen by each individual plate weld is to conduct a 3D “bolt” group analysis. Instead of bolts, small sectors of area going along the weld length could be used. Such an analysis will give you x,y,z forces at the centre of each area, which can then be used to derive distributed loads along the length of each weld. It’s not a perfect method, but a possible method!