Two wires for both digital data transmission and energy supply at the same time / Used anywhere?
Examples? There are 1-Wire, AS-Interface, and others, and this TI application note describes a way of doing it with several possible two-wire connections.
And yes, it is used often.
1-Wire pulls the line low for very short periods; the longest is the reset pulse (480µs or more). 1-Wire slaves use the 1 state (which is also the idle state) to charge a small capacitor for operations while the bus is in a 0 state. The capacitor is in the order of 800pf, according to this design guide. There's a simplified schematic of a slave's internals on page 17 showing the parasitic power supply. If there is enough idle time, you could charge a larger (external) power buffer for the slave, as long as you stay within specs.
AS-Interface works differently, and doesn't interrupt the power supply when sending data.