It wouldn't be ideal, but if they are intent on doing it on the cheap (again) by just building behind it, a two-tiered stand would look a lot better aesthetically than the visuals put forward. And it would provide better views from the new seats.
Atmosphere is certainly another key consideration, and i take your point that having two tiers may not be the ideal solution from this perspective. But issues with singing as one surely just go with the territory for big stands. I'm not sure how tangible it'd be to say a two-tiered stand would worsen atmosphere, especially when full. And if it was half empty then atmosphere would suffer regardless of design.
Of course having two tiers wouldn't be a 'Kop', and if we're precious about this as a club then that is perhaps why it is proposed the way it is. These were McCabe's plans and maybe HRH would have a different vision.
The ideal would be to build an entire new stand, with a steep single tier and safe standing.