The joint distribution of sequences (f_\ell(P_\ell(n)))_{n\in\Bbb N},\ell=1,2,\dots,d and (f_\ell(P_\ell(p)))_{p\in\Bbb P} respectively, where f_\ell are q_\ell-additive functions and P_\ell polynomials with integer coefficients, is considered. A central limit theorem is proved for a larger class of q_\ell and P_\ell than by Drmota~[3]. In particular, the joint limit distribution of the sum-of-digits functions s_{q_1}(n),s_{q_2}(n) is obtained for arbitrary integers q_1,q_2. For strongly q-additive functions with respect to the same q, a central limit theorem is proved for arbitrary polynomials P_\ell with the help of a joint representation of the digits of P_\ell(n) by a Markov chain.