We have developed a tandem-bicycle ergometer, allowing two riders to synchronize pedalling movement sharing one load. We summarize fundamental data concerning cardiorespiratory responses during submaximal and maximal exercise by means of the tandem-bicycling ergometer, compared to those obtained using a single-bicycle ergometer. Heart rate (HR), oxygen uptake (VO2) and ratings of perceived exertion (RPE) showed no significant differences. Observed coefficient of variations (CVs) ranged from 2.8% to 7.3%. To clarify whether two riders shared one load equally up to maximal exercise and whether exact maximal physiological responses of two riders are obtained during tandem-bicycle exercise, maximal values during tandem-bicycle exercise were compared. When variables were compared among the three conditions, no significant differences were noted. CVs were 2.2 ± 1.0% for peak heart rate (HR peak) and 3.1 ± 1.6% for peak oxygen uptake (VO2 peak). Finally, we examined cardiorespiratory responses during maximal tandem-bicycle ergometer exercise in riders at different levels of physical fitness. Time to exhaustion of the low-VO2 peak participant was longer in tandem-bicycle exercises, despite the VO2 peak of a pair of riders differing by 10%. An advantage of the tandem-ergometer is significant extension of the time to exhaustion of the rider with lower peak VO2 given a difference of 10%. From the perspective of cardiorespiratory responses to submaximal and maximal exercise, the newly developed tandem-bicycle ergometer can provide an equal exercise load to submaximal and exhaustion to double riders, at least when the physical fitness of a pair of riders is very close. We speculate that the tandem-bicycle ergometer could improve not only health promotion but also athletic ability.