Avantone mix cube just one?

I have sent back my little Genelec 8010s, and ordered 8040s as main monitors, I am selling my old main monitors (Presonus Eris8)

now I am looking at getting an Avatone mix cube to check mixes, is one best? I have a Plug in which changes the output to Mono.
Or is best to have 2?

Avantone mix cube just one?