I have installed and setup DXA 2.1 .NET sample application and able to run the sample application. Now I have created a new publication at the same level (400 example site level with same parent publications) in cms, how can I map this publication to the same sample application, so that I can publish new publication items and use on the same website url?
It's not possible to use the exact same url for two mapped publications. This way, DXA would never know which publication to use to read content from. You can point two publications to one dxa web app, but then you need to either use a different base url (eg http://english.mysite.com and http://french.mysite.com - if you want to localize by language, or whatever works for you).