Two experiments are reported in which monolingual and bilingual participants who were younger (Experiment 1) or older (Experiment 2) adults performed in a dual-task classification paradigm. Visually (primary task) and auditorily (secondary task) presented stimuli were classified into two categories. The stimuli belonged to two domains, letters or numbers (LN) and animals or musical instruments (AM), and the two task modalities used stimuli from the same (related) or opposite (unrelated) domains. For both age groups, bilinguals were less disrupted than monolinguals in visual sorting in the LN domain, and relatedness affected performance only in the AM domain. The results indicate different processing demands for the two domains, with the simpler domain (LN) favouring bilinguals and the more semantically complex domain (AM) favouring related judgements for the two task modalities.