Fast switching (microsecond to nanosecond) voltage using 10V signal pulse
Connect the 20 V supply with a series Schottky diode, then a transistor between the 50 V supply and the load.

When the transistor is off, the load is powered from the 20 V supply.  When the transistor is on, the load is powered from the 50 V supply.  At that point, the diode between the two supplies is reverse biased, so the 20 V supply is essentially cut off.  There is guaranteed to be no off time.  The switchover time is how fast the transistor can be turned on, and what dV/dt it can produce or tolerate on its output.

For the transistor, a P channel MOSFET is probably the best choice.  A circuit takes the 10 V signal and converts that to a FET gate drive signal.