This is how I see it, they just can't fit everyone in around the stadium for nearly 2 years if they have to close the NS completely. At the very best it would mean virtually no matchday ticket sales. Maybe those that really want it could offer to suspend their season tickets during the build, but there'd certainly not be enough volunteers.
I think a two-tier stand will be mighty impressive if that's what we end up with, but personally I'm okay with a mirror of the South Stand if that's what it has to be.