In Musicbox? No, see Multi-room solution
Aside from the lack of synchronised multiroom support in Musicbox, Spotify allow playback of only one song at a time from an ‘online’ account. And Mopidy itself supports playback of one track at a time. To play different tracks on each speaker you’d need a Mopidy server running on each machine, with either multiple Spotify accounts (£££), using Spotify in offline mode (not currently possible), or by streaming from less restrictive music backends such as youtube/soundcloud/tunein etc. Then you’d have to swap between servers in your client, some clients make this quite easy to do.