Is it 2 students that go up for 3+ hours or are they short flights?
Usually we go up in pairs. Your first flights in contacts (C4101-6 and C4201-2) are onwing, with the first block being 2 hour hops per student, so you're in the plane for ~4 hours at a time. Once you're in the second block of contacts, it goes down to 1.5 hours per stud and stays that way pretty much until Review Stage, when it pops back up to 2 hours. Take water and something to eat.
That said, well over half my flights in instruments were me and the IP alone, which got me very used to not having an observer to help me out (and they can, big time).