As someone said, Megan Sikora covers both Glinda and Nessa, and Shoshana (when she was a standby) covered Elphaba and Nessa (but only performed as Nessa once). I think because the actors they hire to play Glinda (blonde and bubbly) and the actors that play Elphaba (brunette, ethnic-looking) are so different, I doubt there will ever be a standby to cover both those parts.
"Free from what, Tracy? Free from what?"