If you are going to use Rufus because of the 4GB FAT32 filesize restriction, you could still use FAT32 after splitting the install.wim using DISM with the option Export-Image and the argument SWMFile.
Officially your not meant to have multiple partitions on a standard USB key so diskpart doesn't work properly in this instance. You would need a 3rd party partition utility. WindowsToGo USB keys have a bit set so they are seen like a hdd so partitioning is allowed.
Supported Operating Systems: Windows 7, Windows Vista, Windows XP, Windows 2003, Windows 2000 On Windows 10 x64 seen the USB once, but no partitions, so I think the OS is really NOT supported
With Eassos PartitionGuru is possible view and save the entire content of the rufus partition on a folder on your pc, not possible mounting the partition.
Starting with insider build 14965 it is possible to format a removable USB drive with more than one partition...