SAM & OAM between different electrons?

when we consider the coupling of angular momentum between different electrons, we tend to only take L_1 & L_2 coupling, L_1 & S_1 coupling, S_1 & S_2 coupling and L_2 & S_2 coupling into account. Thus, I was wondering why we don’t consider the L_1 & S_2 coupling and L_2 & S_1 coupling? Are them too small so we just neglect them? how to calculate their value?