In real life, all the shims are actual parts with partnumbers that should be modeled as separate CATPart files. A sub-assembly should be made of the longeron and all the shims. And the RH version of the sub-assembly can be made my using the Symmetry tool in the Assembly Design workbench (where you would define which parts are actually mirrored, and which parts can be re-instanced by rotating or translating them). This is what real Engineers/Designers do when designing a real airplane with CATIA.

But.... you are a student doing a quick little project just for fun and without having to worry about things like manufacturing and BOMs and revisions, so you can take some shortcuts. Based on your attachment, I would boolean assemble all the shims into the longeron so you endup with a single partbody of the LH assembly. I would publish the partbody, and then copy and paste special with link the partbody into a brand new CATPart, and then apply a symmetry to make a RH assembly. (if you modify the LH assembly, the RH changes also).