Extruder temperature failure
The first extruder unit appears to have an error the firmware can't detect, which is why TEVO thinks the issue with that extruder is the heat sensor. If the sensor's reading hot relative to actual temp, the block will be too cool to reliably extrude, while the printer won't think anything's wrong and won't try to heat the block more.

However, I'm surprised the extruder can push any amount of any type of fil through the nozzle at room temp, which makes me think there's also some user error here. Non-contact temp gauges like IR guns can be bad at accurately checking very localized temperature; I'd recheck the extruder at what the printer thinks is working temp, but use a contact thermometer like a multimeter with a thermocouple (the Extech MN35 is inexpensive and works well for most hobbyist uses).

The second extruder has a detectable error. Either the sensor is again faulty, but this time reading 0&nbsp;V (which usually equates to 0&nbsp;&deg;C), or the heating element is faulty and can't warm the block. Either way, the printer can command as much extruder heat as it wants, the sensor never gets above room temp much less to operating temp, so the printer eventually gives up and tells you there's an extruder heating problem. 

This printer-detected heating error is a little easier to troubleshoot because the printer can help you. If the hotend gets to working temp as measured externally, but the sensor reads well below it, the sensor is the problem. If the hotend does not heat, or heats very poorly (like only getting into the 150-ish range when trying to heat to 200), it's the heating element. Both the heater and sensor could be faulty, but the combination of two failures is less likely by simple probability, and it's easier to diagnose the sensor issue with a working heater, so if the heater's not heating, fix that first.

It's possible, even probable, that a combination of parts from the two extruders will give you a working hotend, however you have to know which parts are working, and those parts have to be separable (as in you can remove the needed part from one assembly and put it in the other without destroying the part or either assembly), which at this pricepoint is honestly not a guarantee. The info provided in the question isn't quite enough to say which pieces of each assembly should be combined to give you a working extruder head, but the above tests will tell you.