I saw an understudy once in a show that had two female characters who, similarly to the case you mention, were meant to be strongly contrasted. The very competent understudy for both roles didn't seem altogether right for either role. She wasn't bad, but she wasn't really right. (Maybe she would have been at least a bit more right in the other role.) This was Off-Broadway and I guess they just couldn't afford separate understudies.
