WARNING: This is an UNSUPPORTED feature.

Based on the UI Design and Interaction Guide for Windows Phone 7, double-width tiles are reserved for Microsoft only:

Double-width tiles are only available to Microsoft, phone manufacturers, and mobile operators.

I still wanted to at least try it, so I dug around a little and found a hack. Open your WP7 applications WMAppManifest.xml and change the TemplateType5 to TemplateType6.

<?xml version="1.0" encoding="utf-8"?>
<Deployment xmlns=http://schemas.microsoft.com/windowsphone/2009/deployment 
	AppPlatformVersion="7.0">
  <App xmlns="" ProductID="{96081999-0c65-4b70-a88b-15bc04a387ca}" 
	Title="Podium" RuntimeType="Silverlight" Version="1.0.0.0" 
	Genre="apps.normal" Author="CodeProject.Podium.WP7 author" 
	Description="Sample description" Publisher="CodeProject.Podium.WP7">
    <IconPath IsRelative="true" IsResource="false">ApplicationIcon.png</IconPath>
    <Capabilities>
      <Capability Name="ID_CAP_GAMERSERVICES" />
      <Capability Name="ID_CAP_IDENTITY_DEVICE" />
      <Capability Name="ID_CAP_IDENTITY_USER" />
      <Capability Name="ID_CAP_LOCATION" />
      <Capability Name="ID_CAP_MEDIALIB" />
      <Capability Name="ID_CAP_MICROPHONE" />
      <Capability Name="ID_CAP_NETWORKING" />
      <Capability Name="ID_CAP_PHONEDIALER" />
      <Capability Name="ID_CAP_PUSH_NOTIFICATION" />
      <Capability Name="ID_CAP_SENSORS" />
      <Capability Name="ID_CAP_WEBBROWSERCOMPONENT" />
    </Capabilities>
    <Tasks>
      <DefaultTask Name="_default" NavigationPage="MainPage.xaml" />
    </Tasks>
    <Tokens>
      <PrimaryToken TokenID="CodeProject.Podium.WP7Token" TaskName="_default">
        <TemplateType6>
          <BackgroundImageURI IsRelative="true" IsResource="false">
				Background.png</BackgroundImageURI>
          <Count>0</Count>
          <Title>Podium</Title>
        </TemplateType6>
      </PrimaryToken>
    </Tokens>
  </App>
</Deployment>

Once that is done, change the Background.png resolution from 173px x 173px to 356px x 173px and run the application. Pin it to the start and…

VERY IMPORTANT: Remember that your application will not be accepted into the marketplace if you use a double-width tile!!!

Also Read

CodeProject

Double-width Tiles