I want a merger for two wordpress websites to replicate changes.

i am confuse what actully you wan to do. please tell in breif.

if you want to mearge two site you must have to export all you post and pages thrugh WP-ADMIN-> TOOL -> EXPORT then export what you want to merge into the other bolg .

After exort go into the admin panel of other blog and imrot that content there.

Its not possible, there is a wayout, if you prepare a local copy for your website and update on the local copy after successfuly updation you can upload the same changes on both websites.

I create clones for my own and client sites all the time. It's especially important to test final changes to any site on the live server as moving a site from a test server to another live server can sometimes get very messy.

I generally create a new folder on the same server and clone WordPress to that folder. I use buddypress, which is  apaid plugin, managewp also a paid multi site management tool, infinitewp something similar to managewp. If those don't work I move on to XCloner and finally I will do it manually. Each server has it's own unique setup which may cause problems in the backup, clone and restore process. With a dev site on the live server you can easy switch the dev site live and the old site off by moving the old site to it's own old folder and changing the dev site's LIVE url as the previous commenter suggested here - http://codex.wordpress.org/Changing_The_Site_URL

I use a members only plugin to keep the dev site login only.

Atif's suggestion of exporting and importing to merge the2 site's data will work but you need to be careful. It depends on what you want to move as export also carries with it some site settings and not just posts and pages.

How you will do what you want to do depends entirely on the finer details you want to achieve so yes much more detail is needed to give a more accurate answer.

why do you need to duplicate a website and it content in the firstplace. if you ment for testing and development purpose. you can create a local copy of the site on your PC or server and clone it to the proper live server. this you can do in many ways. if you are a newbe use a plugin. 

eg- http://wordpress.org/plugins/wp-clone-by-wp-academy/http://wordpress.org/plugins/duplicator/

or else  the best way to do is upload all the files to ftp server and update the Database to the live server. and follow the steps. in below linkhttp://codex.wordpress.org/Changing_The_Site_URL

FYI.

if it is a wordpress issue first do check in http://codex.wordpress.org you can find answers easily.