how to implement layout

As per your requirement you want to show different layout, but dont want to create different layout page.
It is possible if you have any property of your current user object which shows the client then on your layout page simple you can use code to check the current client and write your different layout structure for that client.

@if (User.CurentClient=="ClientOne")
                        { 
<div id="header>
<ul>
<li>Home</li>
<li>About</li>
</ul>
</div>
<div id="Main">
 @RenderBody()
</div>

}
else if(User.CurentClient=="ClientTwo")
{
<div id="header>
 <span>Title one>
</div>
<div id="otherDiv"><span>any other div Conent</div>
<div id="Main">
 @RenderBody()
</div>

}
else
{
//this will your default arrangement of div and other element on layout.
<div>
@RenderBody()
</div>
}

In this code you simply checking the current user client and Creating Layout for that client and rendering views in that layout.
other things like js,css can also be be separated as the above.

use this code in _Layout.Cshtml