Measuring high voltage in millimeters and other HV probe tricks

Ummm, no...that is, unless your intent is to blindly explore the catastrophic failure modes associated with dielectric breakdown.

Theoretically, yes, but only if the HV meters are ideal resistors. In practice, voltage ratings cannot be added for good reasons.

The safe and legal way is building a separate HV probe for the meter, internally, it can use a resistor divider. The individual resistors will have a much lower voltage rating, and still, they cannot be added, you don't get a new rating by connecting them in series. But you can test and certify the unit as a whole, so the probe itself gets a new input and output voltage rating.

Another example is oscilloscopes. Almost all scopes are only rated for CAT II measurements, which meant it's only safe to use the scope to measure the voltage inside an appliance plugged in an outlet that is far away from the electricity entry panel (even when the working voltage rating is the same, different Categories [0] have different overvoltage handling ratings for unexpected transients, the closer to the entry, the higher it needs to be). Technically, you cannot even use an oscilloscope to measure the AC lines directly - it requires CAT III certification (yeah, most people keep breaking this rule). However, what you can do is building a standalone, differential, active HV probe as your "input front-end", and design and certify the probe to handle the hazardous voltage, in this case the oscilloscope only samples its safe output and acts as a data acquisition and analysis system.

[0] https://en.wikipedia.org/wiki/Measurement_category