In multisensory research, faster responses are commonly observed when multimodal stimuli are presented as compared to unimodal target presentations. This so-called redundant signals effect can be explained by several frameworks including separate activation and coactivation models. The redundant signals effect has been investigated in a large number of studies; however, most of those studies are limited to the rejection of separate activation models. Coactivation models have been analyzed in only few studies primarily using simple response tasks. Here, we investigated the mechanism of multisensory integration underlying go/no-go and choice responses to redundant auditory-visual stimuli. In the present study, mean and variance of response time, as well as accuracy rates of go/no-go and choice responses were used to test a coactivation model based on linear superposition of diffusion processes (Schwarz, 1994) within two absorbing barriers. The diffusion superposition model accurately describes mean and variance of response times as well as the proportion of correct responses observed in the two tasks. Linear superposition seems, thus, to be a general principle in integration of redundant information provided by different sensory channels and is not restricted to simple responses. The results connect existing theories on multisensory integration with theories on choice behavior.