If they were my dogs, I would not have Sadie view Abby. She will only get stressed out and confused by seeing you upset (and probably crying) Instead, I would try and spend a lot of time with Sadie, and keep her busy, which will help her get over Abby being gone. Whatever you do, I wish you luck and am sorry about Abby.
My heart aches for you and your family..
You will know when the time is right to let Abby go, as she will let you know as well.
Last September 2011 our wonderful Dixie Carter, a mini dachshund rescue, passed away in my husbands arms as he was walking her into the Vets office... The prior owners never had her 'kennel cough' treated, so after years of her coughing so hard, her treachea gave out. We decided to do just what you mention here. We put her in her favorite bed with her blanket, wrapped her up just a bit, and let the other doxies pay their respect. They all had a different reaction, but none were negative. It was actually very sweet the way they softly sniffed and nuzzled her.
With you going from two to one, it may be a bit more difficult for Sadie as she will be only one, but I believe you can help fill her void till she feels comfortable again.
Hope everything goes smoothly for all of you!