  We demonstrate simultaneous electromagnetically-induced transparency (EIT)
with cesium (Cs) and rubidium (Rb) Rydberg atoms in the same vapor cell with
coincident (overlapping) optical fields. Each atomic system can detect radio
frequency (RF) electric (E) field strengths through modification of the EIT
signal (Autler-Townes (AT) splitting), which leads to a direct SI traceable RF
E-field measurement. We show that these two systems can detect the same the RF
E-field strength simultaneously, which provides a direct in situ comparison of
Rb and Cs RF measurements in Rydberg atoms. In effect, this allows us to
perform two independent measurements of the same quantity in the same
laboratory, providing two different immediate and independent measurements.
This gives two measurements that helps rule out systematic effects and
uncertainties in this E-field metrology approach, which are important when
establishing an international measurement standard for an E-field strength and
is a necessary step for this method to be accepted as a standard calibration
technique. We use this approach to measure E-fields at 9.2~GHz, 11.6~GHz, and
13.4~GHz, which correspond to three different atomic states (different
principal atomic numbers and angular momentums) for the two atom species.
