You could use a line of JS to check the width of the screen and then direct to a mobile version if screen narrower than xx - or the other way round.

You could instead have a nice one-page site, that is good in both desktop and mobile.

On that page you tell the user that he/she can choose between a and b, and that if he does not choose it will happen automatically in xx seconds ( and then redirects according to screen width)

Then you have the A and B projects in subfolders. In each project you could have a menu item that leads to the a project if you are in B, and to the B project if you are in the A project.

Making it this way will let the user choose what he prefers. Maybe he does not want the site to change if he tilts his device from portrait to landscape - or the opposite.

Thanks for the guidance 

I was found the following code and i test it working perfect to android also to iphone 

Andoid

<script type="text/javascript"><!--if(screen.width <=699){ document.location ="mobile.html";}//--></script>

Iphone

<script language=javascript><!--if((navigator.userAgent.match(/iPhone/i))||(navigator.userAgent.match(/iPod/i))){ location.replace("http://url-to-send-them/iphone.html");}--></script>

Esahc,

Yes, you have right .... now I see carefully the entry page and the option for the viewport...

I will make it and test it.

Thanks