Actually, Lingvo is even smarter than I said, so I need to correct myself: as soon as you launch Lingvo, it not only inserts whatever is in iOS's clipboard into Lingvo's search field, but it also immediately displays the definition and/or translation of whatever is in the search field. And so, you don't need to tap anywhere in Lingvo at all, not even once. You do need to perform the four-finger swipe to return from Lingvo to Marvin.
(Lingvo also displays alternative/close-match search suggestions related to whatever is in the search field, so obviously you need to tap a suggestion if the definition/translation of the default search term is not the one you're looking for.)
If this was the world of Mac OS or Windows, the ideal solution would be obvious: simply overlay Marvin's full-screen window with a small Lingvo pop-up window, and you get the same effect as with iOS's built-in dictionary. However, this is likely impossible in the full-screen world of iOS, so it looks like that "round-trip" to an external dictionary app is inevitable.
