Each Visual Studio instance started with the RootSuffix parameter keeps its own set of extensions. If a Visual Studio extension is available from the Visual Studio Marketplace, you can just install it from the standard Manage Extensions dialog. But if you have only a .vsix file you can use vsixinstaller like this:

"C:\Program Files (x86)\Microsoft Visual Studio\Installer\resources\app\ServiceHub\Services\Microsoft.VisualStudio.Setup.Service\vsixinstaller.exe" /appidinstallpath:"C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\devenv.exe" /skuName:Community /skuVersion:16.11.31624.102 /appidname:"Microsoft Visual Studio Community 2019"  /culture:"en-US"  /noep  /rootSuffix:"Demo" TabsStudioVS2019.vsix

Note that example parameters are for the Community edition.

And to find correct skuVersion you can run

"%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe"

and copy installationVersion property value from its output.