[Tostino]

At least Totoro was the latter movie in the pair so you didn't go home feeling terrible.

[savoyard]

Not always. Here is Isao Takahata [1]:

“The response was different depending on which film was shown first. My Neighbor Totoro would make them happy, then this Grave of the Fireflies… Those who saw Totoro first didn’t want to see Fireflies to the end. Those who saw Fireflies first didn’t have that problem, and stayed to the end. The double featuring was a problem, I’d say.”

[1] http://www.nausicaa.net/miyazaki/grave/faq.html#totoro

[hinkley]

If you just watched Grave of the Fireflies, would you trust the second movie?

Also the crises in Totoro would be a lot heavier if you just watched a whole family die piece by piece.