Dear Hassan,
The article sharing is perform with the "Article sharing for JMS" extension.
See product detail
www.jms2win.com/download?page=shop.produ...34&category_id=1
If you have this "article sharing" extension, you just have to install it in the s2 website to be able see the article from s1
When you will define the menu item in the s2 website, select the multi site article and there you will be able to select the site ID in addition of the article as you do with the standard Joomla.
When the article sharing is installed in a slave site, it is able to see ALL articles of all the website defined in JMS.
Today, there is no restriction "Article Sharing for JMS" concerning the website where it can collect the articles.