Powering a microcontroller circuit using a differential voltage instead of an absolute voltage
Yes you can, in fact, people do it every day. Often a mains powered H-bridge will use an isolated high side gate driver, so running at 340v and 15v above that.

What people do is implement the two different voltages by setting one relative to the other, for tolerances and for practicality.

Tolerance. Consider that you had a 10kV power supply and a 9995v supply, and that they were both very very good tolerance, say 0.1%. Their difference would be 5v +/- 20v, anything from -15v to +25v. Not a nice environment for your 5v chip!

Practicality. Why make two different standalone 10kV power supplies, twice as expensive as a single supply, when most of the time what's needed is a cheap 5v supply 'riding on' the 10kV.

For a low current supply, your 10kV supply could use a current leak to ground through a big resistor, and use a 5v zener to regulate the 9995v with respect to the 10kV.

For any serious power at 10kV, it would be less wasteful to use an isolation transformer, to supply a rectifier and 5v regulator at 10kV.

The mains powered high side driver often uses a charge pump capacitor, as it doesn't need full isolation from the low side, and needs a reasonable amount of power.