Final answer:
The single that features two Beatles innovations: backtracking and varispeeding is "Strawberry Fields Forever".
Step-by-step explanation:
The single that features two Beatles innovations: backtracking and varispeeding is the song "Strawberry Fields Forever". Backtracking is the process of recording a new track over an already-recorded track, creating layers of sound. Varispeeding is the technique of altering the speed of a recording, which can change the pitch and tone of the music.