The use-first-or-wait-two-hours applies to your *most recently booked* return time. So if at 2:20 they book a 3:15 G+ as their third choice, they'd be able to book a 4th after they use the 3:15 one, even if they haven't used #'s 1 & 2.
Theoretically, if they have wifi on the plane, they could...