It seems that the Loeb volume mentioned in the subject should have a "Tacitus" index, but it does not. The Latin volume does. Even the dead tree version would offer facing pages that would essentially function the way I would like for the Multiple Book Display to work, but without a meaningful index, I don't think it's possible.
I would even offer to do the work. :)
Shouldn't the Loeb volumes both (the original language and the translation) have a similar functioning index?