Hi @pradeep_07 ,
Visitor ID service should serve the same MID as long as you are using same Experience cloud ORG ID in the configuration and the demdex cookie is present, i.e. when third party cookies are allowed in browser.
To achieve cross domain tracking when third party cookies are blocked you can implement appendVisitorIDsTo function ( https://experienceleague.adobe.com/docs/id-service/using/id-service-api/methods/appendvisitorid.html?lang=en#id-service-api ) which should fire on the link call which take the user from one site to another (cross domain site link) A to B.
This method works only when a visitor navigates from one domain to the other via any link click and not directly opening the other domain in new tab.
You can also visit this link to understand how cross domain tracking works: https://helpx.adobe.com/marketing-cloud-core/kb/MCID/CrossDomain.html
Hope this helps !