Web Packages created based on the above walkthrough (i.e. via VS 2010 and its derivatives like TFS etc) can only contain one web project. Unfortunately VS generated Web Packages will be limited from this sense. If you use Web Deploy (MSDeploy) EXE or API directly and then you can package more than one web site within IIS etc. I believe you are trying to accomplish this via Team Build so that is not really an option.
Although, in Team Build you can create more than one build definitions to cause multiple packages to be created. You can also set properties in your .csproj or .vbproj file (same properties mentioned in Step10 of the walkthrough). If more than one project have properties DeployOnBuild set to be true then from within single solution build within TFS you can have multiple web packages generated. You will still have to deploy each web independently.
Hope this helps