SEO Consideration when Switching Domai

Selected answers from the Dumb SEO Questions Facebook & G+ community.

• 
  Ankit patel: Client have  ;2 domains for their services.  ;Both have same content and same pages, if we change to one site it will also reflect to other site.
  
  But the issue is now client wants to target new domain for SEO
  For SEO They Choose ; ; ; Newdomain.com which is not already index in Google.
  
  I suggested them to redirect ; ; ; olddomain.com to new.com ; ;  ;but they have issue with that
   ;
  Is there any alternative solution to index ; ; ; new domain  ; ;in search Engine? (Currently Google index Old.com ; ;
  
  If I add disallow robots.txt to ; ; ; www.olddomain.com ;  ;it automatically add to www.newdomain.com
   ;
  Can I suggest them to add canonical tag on all pages of ; ; Newdomain.com
• 
  Roxanne H: Very interested to see answers as we have the same circumstances about to come about with our new web-site. Should we keep old site running while we build other site or redirect to new site to get things going quicker with the new site? We are worried probably like others we will loose too much in sales waiting on the ranking to kick in. ;
• 
  Ankit patel: in my case i have not option of 301 redirection , i have to kee both domain running but  ;SEO will be apply on new website.