Todd,
From Dynon's perspective:
You could share EGT / CHT data between two EMS units. These will not be affected by running to two units. The comment in this thread that as long as the gauge is a powered thermocouple gauge it can be shared is generally correct.
Any sensor that is resistive cannot be shared. So resistive fuel levels, OAT, oil pressure, oil temp, and fuel pressure can't be shared. The EMS sends current out to these sensors, and if you hook up tow EMS units, you get double the current and both units read wrong.
You may also have the issue that the G3X uses quite different sensors than a lot of the other experimental EFIS vendors do, so getting those working with the GRT may cause issues as well.