Zero-error coding encompasses a variety of source and channel problems where the probability of error must be exactly zero. This condition is stricter than that of the vanishing error regime, where the error probability goes to zero as the code blocklength goes to infinity. In general, zero-error coding is an open combinatorial question. We investigate two unsolved zero-error problems: the source coding problem with side information and the channel coding problem. We focus our attention on families of independent problems for which the probability distribution decomposes into a product of probability distributions. A crucial step is the additivity property of the optimal rate, which does not always hold in the zero-error regime, unlike in the vanishing error regime. When the additivity holds, the concatenation of optimal codes is optimal. We derive a condition under which the additivity of the complementary graph entropy $\overline{H}$ for the AND product of graphs and for the disjoint union of graphs are equivalent. Then we establish the connection with a recent result obtained by Wigderson and Zuiddam and by Schrijver, for the zero-error capacity $C_0$. As a consequence, we provide new single-letter characterizations of $\overline{H}$ and $C_0$, for example when the graph is a product of perfect graphs, which is not perfect in general, and for the class of graphs obtained by the product of a perfect graph $G$ with the pentagon graph $C_5$. By building on Haemers result for $C_0$, we also show that the additivity of $\overline{H}$ does not hold for the product of the Schläfli graph with its complementary graph.