It is possible. I think it is called as side by side 
execution. But i don't konow the code. If any one know 
please write.

Off course Krishna is right it is called as side by side 
execution
<Configuration>
<runtime>
<assemblyIdentity name="ABCD" publicKeyToken="Sometoken"
culture="neutral" />

<bindingRedirect
oldVersion="1.0.0.0-99999.99999.99999.99999"
newVersion="1.0.1445.38584" />

<codeBase version="1.0.1445.38584" href="C:\Program
Files\TPA\ABCD.dll" />

</dependentAssembly>
</runtime>
<Configuration>

OfCource sachine ,what u had written is correct,

if u adding System.Web.ui namespace, Namespace contain more 
than one dll also,that Each dll has its own version and 
they are now runnig in same time.

<add assembly........> is one dll with version xyz....

in the below config file there are 8 dll which has its own 
versions  runnig at the same time  





 <compilation debug="true">
      <assemblies>
        <add assembly="System.Core, Version=3.5.0.0, 
Culture=neutral, PublicKeyToken=B77A5C561934E089" />
        <add assembly="System.Web.Extensions, 
Version=3.5.0.0, Culture=neutral, 
PublicKeyToken=31BF3856AD364E35" />
        <add assembly="System.Data.DataSetExtensions, 
Version=3.5.0.0, Culture=neutral, 
PublicKeyToken=B77A5C561934E089" />
        <add assembly="System.Xml.Linq, Version=3.5.0.0, 
Culture=neutral, PublicKeyToken=B77A5C561934E089" />
        <add assembly="System.Web.Extensions.Design, 
Version=3.5.0.0, Culture=neutral, 
PublicKeyToken=31BF3856AD364E35" />
        <add assembly="System.Design, Version=2.0.0.0, 
Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
        <add assembly="System.Windows.Forms, 
Version=2.0.0.0, Culture=neutral, 
PublicKeyToken=B77A5C561934E089" />
      </assemblies>
    </compilation>