Here the second phrase clarifies that "the tablets of stone" are the ones on which God wrote the Ten Commandments. (See: [[rc://en/ta/man/translate/figs-parallelism]])
The abstract noun "assembly" can be expressed as the verb "gather together." AT: "on the day when you Israelites all came and met together in one place" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])