Recent evidence suggests that the degree of interference in dual-task situations depends crucially on the pairings of input- and output modalities of the two component tasks with increased dual-task costs for modality incompatible (i.e., visual-vocal and auditory-manual) compared to modality compatible (i.e., visual-manual and auditory-vocal) dual tasks. These effects of modality pairings in dual tasks have been related to the overlap of non-preferred processing pathways in modality incompatible tasks. Until now, modality compatibility has not yet been related to other sources of interference in a dual-task context, such as stimulus-response (S-R) compatibility or crosstalk. In the present study, we conducted two experiments using the paradigm of the psychological refractory period (PRP) to test the effects of S-R compatibility and crosstalk on the effects of modality compatibility in temporally overlapping task situations. Experiment 1 revealed an overadditive interaction between stimulus onset asynchrony and modality compatibility for tasks with S-R compatible mappings, indicating that modality compatibility effects are present in different task situations, even when S-R mappings are otherwise compatible. In Experiment 2, we aimed at pinpointing the boundaries of the effects of modality compatibility in dual-task situations. We showed that additional sources of dual-task interference in a modality compatible dual task could overwrite the pronounced PRP effect previously shown for modality incompatible tasks. Taken together, these data provide new evidence that the specific types of stimulus-response modality pairings are an additional factor that might interact with other sources of interference in dual-task situations.