Cognitive-motor interference research investigates how balance control demands affect cognitive task performance and vice versa. In a recent study, balance control parameters were measured continuously with a force plate during quiet standing and analyzed in an event-related manner. The results show that single cognitive operations that resolve response conflict in the Simon task reduce variability in balance control prior to the manual response. Here, we used this event-related methodology during a cognitive dual task. This dual task consisted of a visual short-term memory task with a delayed verbal response and an auditory-manual reaction time (RT) task. We manipulated the stimulus onset asynchrony (SOA: short vs. long) between the visual and auditory stimuli, and whether participants had to report or ignore the visual task (task load: single vs. dual task). Manipulations were applied on a trial-by-trial basis or in blocks of predictable versus unpredictable task load. RTs in the auditory-manual task were higher during short SOA, suggesting a bottleneck-like effect. This effect was particularly pronounced for unpredictable task load, possibly due to the maintenance of two task sets for both report (dual task) and ignore (single task) trials. When assessing balance correlates of cognitive conflict for SOA and task load, similar patterns were found during response selection. Specifically, there was increased sway variability during short SOA, which was pronounced in unpredictable blocks. We suggest that during the bottleneck and multiple task-set maintenance, there is an interference with the occurrence of intermittent control impulses governing balance processes, resulting in increased sway variability.