usually there is something for the offtank, or second tank to do, like taunt the boss when the boss applies too many stacks of a debuff to a tank. The only boss I can think of where an offtank has absolutely NOTHING to do is general vezax in ulduar. Basically patchwerk without the need for additional tanks. Then all you can really do is go DPS/heal. Most bosses do, however, have some means of keeping both MT and OT busy.