If the point to your multidomain is to have a mobile and a desktop version of your site, you don't need two domains. Concrete5 has a setting to revert to another theme when a mobile device visits the website. Thus you can design two different themes for the same website. Keep in mind that blocks have to keep the same name and structure etc. for the content to show up everywhere.

I tend to rather try and design responsive themes if needed as I find it simpler to handle a website when everything is in one place.

Having ahttp://testabc.com andhttp://mobile.testabc.com means thathttp://mobile.testabc.com will be in another folder or directory on your server. You could directhttp://mobile.testabc.com tohttp://testabc.com but then you are essentially back to square one as both domain would go to the same place.

If your point is not to have a mobile version of a site but a completely different site and keep one C5 install it becomes tricky and I will rather just build two separate websites.

Thanks Trebxuor, Yes I want a mobile version of my site but the url for mobile version is different. The content of mobile version is also come from main website that means desktop version. I know that we can set different theme for mobile version but how can we manage different url for mobile version?

Please someone help me for this post.

There are two extension for mutidomain:

http://www.concrete5.org/marketplace/addons/multiple-domains/...
http://www.concrete5.org/marketplace/addons/domain_mapper/...

Thanks wasabili...but this are paid addon. Is there any free addon?