setup multisite into multi server I developed multi site using Drupal 7.  


example.com
site1.example.com
site2.example.com


All subdomains are using the same codebase in /sites/site1 and /sites/site2.

Is it possible to host SITE1 in a different hosting server with the same codebase?
You need to read this article:
Multi-site - Sharing the same code base
https://www.drupal.org/docs/7/multisite-drupal/multi-site-sharing-the-same-code-base


Create directories for your each site    
Copy config files to each directory  
Create a database for each site
Make a symbolic link for each site:

$ ln -s . site1
$ ln -s . site2