Question

why are two anodes used instead of one?

Answer 1

The reason is that when the space charge on the cathode is neutral the charge is static, thus placing an anode with a much higher voltage some way from the cathode induces a current to flow from the cathode to the anode.
To maximize the potential, to gain much higher currents, there are other anodes with even much higher voltage potentials above the lower anodes.
Some anodes are called 'control grids'. These grids are often voltage modulated, as with an audio tube, and the grid voltage changes with the induced changes in the grid voltage, and sometimes these can go to a negative potential, too, but these are special cases used in power control circuits.