This discussion may contain spoilers!
After interrogating Toad Jr. the Magic Mirror reveals the whereabouts of Tweedle Dum when asked. Surprisingly, he seems to be inside Crane's supposed green car apparently waiting, certainly not driving.
My assumption is that he is waiting for Tweedle Dee to be released from captivity by Crane. If that is the case, then why is he in Crane's car or could it be that the Tweedles own a similar looking car?
Maybe the reason why Tweedle Dee won't reveal who his boss is because Crane is in the same room as him?