Try substituting "temperature" with "size" or "cost" in the sentence. Would you use the plural or the singular?
As @EdwinAshworth alludes to in the comments, it could be either "temperatures ... limit" or "temperature ... limits" depending on whether the intent is for the reservoirs' temperatures to be regarded collectively ("temperature") or individually ("temperatures").
Consider, by way of analogy, the following:
The choice of the voters is what will ultimately decide the matter.
The choices of the voters are what will ultimately decide the matter.
"Choice" could be singular or plural depending on which perspective the speaker/writer wishes to emphasize.
(Related discussion at Temperature vs Temperatures )
Examples using temperature in the singular:
Measure the temperature of both the turkey and stuffing! (Associated Press article, 2011)
In the females, an analogous vascular net helps regulate the temperature of developing fetuses.
(article in Natural History, 2003)
The refrigerator is well featured in several other respects: Two thermostats control the temperature of the fresh-food and freezer compartments....
(article in Popular Mechanics, 2001)
Model ST250 has been designed to be accurate to +2 percent of full scale when measuring the temperature of boilers, air ducts, motors, bearings, or furnaces up to 120 degrees C.
(article in Mechanical Engineering, 1997)
By measuring the speeds of different waves, astronomers can determine the temperature of different layers because the speed of sound increases as temperature increases.
(article in Astronomy, 1995)
Automated control systems are now commonly used to control thermostat settings, the temperature of chilled water and hot water coils, and the amount of air-flow....
(article in Mechanical Engineering, 1994)
Examples using temperature in the plural:
To ensure that the liquids are being stored safely, the temperatures of the railcars must be checked at least once a day.
(article in Mechanical Engineering, 2008)
Up to this point in the investigation, testing used heaters that simulated the temperatures of computer chips in operation.
(article in Mechanical Engineering, 2001)
At a computer keyboard in a comfortable office, he was recording the temperatures of 15-ton steel ingots immersed in the soaking pits where they are reheated.
(article in the New York Times, 1993)
The temperatures of individual areas of skin were converted into a useful depiction of temperature patterns by jotting each temperature on a gridded diagram of the lower limbs....
(article in Journal of Rehabilitation Research & Development, 1996)
Without further context, I would lean toward using:
The temperature of the hot and cold reservoirs limits the maximum possible efficiency.
under the assumption that the unitary sense of temperature is being emphasized here, and thus temperature is uncountable.