I don't know if this helps, but look at the simulation below. This assumes a dielectric constant of 2 for the BNC Tee and a distance of 46mm between the probes. The simulation is a harmonic balance simulation at 500MHz.
You can see there is about a 2.9dB difference in voltage between the two nodes TP1 and TP2 at 500MHz. The HB simulation assumes steady state has been reached.
It's entirely up to you if you want to declare this difference as 'negligible'. However, I urge you to study it closely. The 10073C has a lower impedance at 500MHz yet it has the higher voltage compared to the higher impedance Zo probe. This happens at equilibrium (steady state) after lots of reflections back and forth. Most people would expect the voltage to be higher at the Zo probe TP2 but it isn't.
You have to analyse how the system achieves steady state in order to understand that there will a 2-3dB difference between TP1 and TP2 at 500MHz.
