The situation is the following - my webmaster has made two domains for website – the main domain and sub-domain one, he told me it is more convenient for him to have 2 domains for the same website! He has written the main domain in robots.txt file! As a result, this domain was displayed in all search engines however the situation has been changed recently again! Now sub-domain is displayed in Google however the main domain continues to be displayed in other search engines! What would you recommend to do my webmaster to solve this problem and to have the main domain displayed in Google!

The first question is: why would you need two websites?
Are they significantly different?
If not, just do a 301 redirect from one to another and your problem will be solved.

he told me it is more convenient for him to have 2 domains for the same website!

I guess he is now finding out that it isn't!

What would you recommend to do my webmaster to solve this problem and to have the main domain displayed in Google!

301 redirect the sub-domain to the main one.

Originally Posted by 3DProf4online

he told me it is more convenient for him to have 2 domains for the same website!

Convenient for him != convenient for you.

Is the content on your subdomain identical to the content on your "main" domain?

If they have the exact same content do as mentioned above and use a 301 redirect, if they have different content then clearly Google likes the subdomains content better anyway.

Yes, I think guys it is better to upload a website under the main domain and problems will disappear as a result! I will kick my web master as much as I can!