If you ignore the error in the thermocouples themselves, a thermocouple logging system will be accurate based on the following:
1) Cold Junction Error
2) Amplification Error
3) Linearity Error
Thermocouples measure the difference between the hot (the casting process) and the cold (data logger) junctions. In order to determine the actual temperature you must know what temperature the data logger is at precisely. You then compensate all the temperature readings against this known temperature. This is called cold junction compensation. The ADC-24 has no on board temperature sensor to determine the amount to compensate. In order to use the ADC-24 you will have to design a circuit to measure the temperature of the unit, and then apply a equation to all the channels to offset their readings. Linearity will also be based off this compensation. You may find that you can get the ADC-24 accurate at certain temperature - yet off by several degrees on other ranges. It will vary based on how accurate your CJC circuit and equations are. The USB TC-08 has the CJC built in already with accuracy specifications and calibration certificates available to back them up. There is a reason why many calibration houses around the world use the TC-08 for testing other devices.
For $130 more for 2) TC-08s compared to 1) ADC-24, me personally, would save my time and purchase a unit built for my project rather than spending time on building a circuit.
Overall, the ADC-24 is a very good unit with features that you can't find in hardly any other unit at its price range. That being said, for your application I still believe you would be happier in the long run with the TC-08's.
Crag Technologies, Inchttp://www.pc-oscilloscopes.com