Currently, Spine does not have a direct feature to mirror weights from one mesh to another along the X-axis. However, you can manually copy and paste the weights by selecting the vertices of the mesh with the desired weights, copying them, and then pasting them onto the corresponding vertices of the other mesh. This process requires that the number of vertices selected when copying matches the number of vertices selected when pasting. Unfortunately, the Weld option won't work in this case because it requires common bones between the meshes, which you don't have due to the different arm setups.
For now, you will need to manually adjust the weights for the right arm after copying them from the left arm. This involves selecting the corresponding vertices and ensuring they are weighted to the correct bones on the right side.
