Is there anyway in which I can maintain two different ember-cli projects with different versions of ember-cli on my system ?? 
The use case is that I want to start a new ember app but don’t want to upgrade ember-cli on an already existing one. 

As @ilkkao said, using the relative path to ./node_modules/.bin./ember does the trick.  I also use smartcd and have a .bash_enter file at the root of every ember-cli project that prefixes `./node_modules/.bin/ to the front of my $PATH.