The Gateway Development Corporation has released a list of "mythbusters" on the new tunnels and the North Portal Bridge that you can read here http://www.gatewayprogram.org/content/d ... chment.pdf. A quote:
Building a single new tube is not practical because it would not comply with critical fire and life safety requirements.
• A single, one-track tube beneath the river would not provide safe exits for passengers in the case of an emergency.
• A two-track tunnel, as proposed for the new Hudson River Tunnel, provides cross passages every 750 feet for the length of the new tunnel, connecting the two separate tubes. With only one tube, there would be no other means to exit in an emergency.