The EGT guage actually reads a difference in the tempertature of two dissimilar metal junctions -- one junction is in your manifold and the other is, well, at the end of the supplied (special alloy) wire from the mfg. some mfgs have a longer lead (distance of the special allow wire between the two junctions) than others. The reason this is important is that the gauge has to be calibrated with an assumption: Namely, the temperature of the 2nd junction, and where that junction located, and the length of special allow wire. Put the 2nd junction in the cab, and you can calibrate the EGT gauge to that. put the 2nd junction under the hood and you have to make a different assumption and you will be less accurate, depending on how the guage is calibrated.
The worst thing about putting the 2nd junction under the hood is that no matter what temperature you assume for calibration purposes, it will be wrong during extreems of temperature, and your gague will be innacurate.
Kieth is right about not connecting two gauges to the same thermocouple, and there are two reasons for this:
1. the gauge face will be calibrated with an assumption of where that 2nd junction is, and its expected temperature. that assumption may not be valid if you use a different mfgs thermocouple. This is why, standards notwithstanding, one mfgs theromocouple may read differently on another mfgs gauge.
3. Keith mentioned the big reason -- it turns out that these dissimilar metal junctions are relatively high impedance devices -- meaning that the reading on the guage face depends heavily on how the guage amplifier is built and calibrated. Add another gauge to the same thermocouple and you have just blown that whole design point away -- and the worst part is that the error is in the WRONG DIRECTION. that is, hanging two gauges off of one thermocouple means that your readings will always be too low by some amount that is hard to predict.