there is a much simpler meathod.
it says corectly time 9 mins with the two glasses, it never says you have to use them at the same time.
so start the 4 min glass, when that is half gone, start the 7 min glass
much faster and much less hassle then to stand there and flip the glasses several times.
Aseatha85 is correct. Never did it mention that you were to time 9 minutes on EACH glass.