Aubrey Whitfield routes every backing vocal, harmonies, doubles, ad-libs, into a single bus so they can be treated as one cohesive element instead of a pile of separate tracks.
On that bus she stacks EQ, compression, and de-essing to glue the parts together and stop any single vocal from poking out of the mix.
For thin or dull-sounding backing vocals, she adds saturation to beef up the tone. A bass-relief setting keeps the distortion on the mids and highs only, since low-end saturation can muddy the vocals too much.
On panning, her rule of thumb is to rarely go past halfway toward hard left and right. In her experience, pan too wide and the vocals start to sound disconnected from each other, and you risk losing them if the mix gets played back in mono.