In yoga, there is this thing called ativyayam. This is over-exertion beyond one's capacity. This imbalances prana and will create disease later. This is also not correlative to any Western concepts of anything and the diseases are not able to be correlated by Western medicine as well due to the lack of comprehension or context of/for prana. One's capacity is seen when the heat inside the body reaches a point where the body can no longer stay cooled by normal breathing thru the nose. This is also why in 'yoga' we breathe through the nose. When the heat increases beyond one's capacity, you will need to breathe through the mouth, then secondarily perspiration starts on the brow and above the lip. These are signs of one's maximal capacity. Any exertion further will result in imbalancing of prana and is detrimental to the health causing disease.
Any increase in physical movement will automatically increase vayu or vata in the system. To do two classes a day when one is not ready to will be detrimental to the health and will also result in disease later. This would be even more nonbeneficial in the summertime when exercise and exertion are to be kept at minimum due to the natural weakness of the human body just due to nature and the seasons. Due to the dryness of the summer season vata is aggravated (not pitta) and this is just even more reason to not do two classes a day.
Another point is that stiffness is also vayu based. More of a reason not to take two classes a day.
Bigger point of this, do not try to put Western concepts on yoga. More is not better. More is just more.
None of the above is understood in the modern yoga world because that is based solely in Western exercise physiology, not yoga physiology, yet what is written above is from yoga based deeper knowledge.
In the future when a teacher tells you something, ask why. Only in doing this do you learn.