In 1989 there was a study that investigated the behavior of animals. Caffeine was anxiogenic in the context of exploration and social interaction while yohimbine was anxiogenic in the context of exploration but displayed anti-conflict properties in social interaction . Unexpectedly, caffeine and yohimbine cancelled each others’ effects out.
An interesting study investigated rat vocalizations under the influence of either caffeine, yohimbine or amphetamine. 22 kHz sounds were interpreted as signals of alarm or distress while 50 kHz voice were interpreted as response to rewarding stimuli while mentioning that it is unknown whether the same signal is associated with anxiety . Caffeine and amphetamine increased 50 kHz calls, yohimbine did not. In addition, it was shown that while caffeine and amphetamine were locomotor stimulants, yohimbine was not.
Yohimbine can promote fat loss by mildly increasing lipolysis when not counteracted by eating because after a meal the lipolysis-enhancement does not work . In lower doses it does not increase blood pressure, heart rate or cause anxiety as much as ephedrine but larger doses have both cardiovascular and anxiogenic effects.
A human cardiology study investigated caffeine, ephedrine and yohimbine in obese women during exercise (handgrip and cycloergometer). Caffeine and ephedrine increased heart ejection fraction during cycling (amount of blood pumped out of left ventricle, an increase may be considered usually beneficial) and did not alter the hemodynamics while resting. Addition of yohimbine increased diastolic blood pressure (that may be considered quite undesirable in this context) and heart rate (same interpretation) but decreased ejection fraction (same interpetation) and stroke index during rest (same interpretation). During exercise, yohimbine decreased ejection fraction during both exercises and increased cardiac load during exercise.
Bottom line: while yohimbine seems useful for fasted long-duration low-intensity fat burning training, it seems that combining it with substantial amounts of caffeine may be asking for trouble. Most importantly, yohimbine seems a very unwise choice for hard exercise (weightlifting, sprint, high-intensity interval training, intense cardio, and, if I may suggest, also pushing the limits with rough sex).