Each WinMount instance can only be signed into one FolderGrid Domain at a time. So to map drives from multiple Domains locally you need to run multiple instances of WinMount on your workstation.
Each instance of WinMount must use a unique mutex to keep it independent of the others. So you must specify a non-default mutex name for each separate instance.
You can do that easily by simply creating a ShortCut to Winmount on your workstations and then adding "-m any_unique_name" to the end of the "Target"
For example the following entry in the Target of a shortcut configures a non-default mutex:
"C:\Program Files\FolderGrid\WinMount\FolderGridWinMount.exe" -m mySecondWinmount