Wow if I had to be ready to play two roles like that, I would really freak out. I wish I could remember J&H better, most of the first act is blurry.
I really love this thread topic! Wish I could think of more examples!
In 1989 Sweeney Revival, Bill Nabel played Pirelli and was stand-by for Beadle
Gypsy- Jana Robbins was Mazeppa and stand-by for Rose
Michael McCormick and William Parry had many featured roles and u/s'd Herbie.
In Mamma Mia, some people have been Eddie, u/s Sky.
In previous version of Sweet Charity, didn't Bebe Neuwirth play Nickie and understudy Charity?
In A Chorus Line, Vicki Frederick (among others) was Sheila, u/s Cassie
Dean: Can I tell you something?
Lorraine: That depends on what it is.
Dean: I think you're really really pretty.
Lorraine: (after a pause) Ok, you can tell me that.