Dual booting Windows with another version or build of Windows has always been pretty straight forward in the past when your device runs on a typical BIOS based system.
However, with the new UEFI aka User Extensible Firmware Interface and BitLocker things have changed a little bit and require particular attention to a few additional steps to make sure your dual boot install is successful.
Personally, I have been fighting this process off and on for several weeks in order to install Windows Insider Fast ring builds on Surface Book and also keep the Current Branch version of Windows 10 up and running for reliability.
I actually upgraded from Windows 10 Current Branch on Surface Book to Fast Ring Build 14955 in late October and it was not a very pleasant experience. The upgrade actually went fine - using that pre-release build on Surface Book was the bad part of the process. So I flattened Surface Book and reinstalled the Current Branch release of Windows 10 to restore everything to the status quo.
However, I still want to be able to test these Creators Update builds on Surface Book to take advantage of the touch screen, Surface Pen and the new Paint 3D app and that meant figuring out how to get dual boot working. That meant starting with the creation of a bootable USB flash drive to install Windows 10 Build 14971 from and making room on the Surface Book's 512GB hard drive with a new partition.
Creating the partition using the built in Disk Management app is straight forward and by default in Windows 10 that new partition is automatically protected with BitLocker encryption.
So the first step once the partition is ready is to switch off BitLocker. Failure to do this will result in an error once you are installing from the bootable USB flash drive.
Open File Explorer to This PC and look for your new partition. Right Click on it and then select Manage BitLocker from the Context Menu.
On the next screen look for your target drive and find the Turn off BitLocker link - just click/tap it to proceed.
Click or tap Turn off BitLocker in the next dialog box.
The next screen will show you that BitLocker is now turned off on that drive and you can now move to the next step - creating your bootable USB media for the installation process.
You want to make sure you already have the Windows 10 ISO for the build you want to install so you can go to the Windows Insider website and download the latest Slow Ring build if you want to test the Creators Update. Once it is installed you can always move to the Windows Insider Fast Ring for future Fast Ring builds.
The secret sauce to building the right bootable media for installing in this second partition on Surface Book is to use this great third party media creation tool called Rufus.
Once you have downloaded Rufus no install is necessary so just select the executable and then setup the app like shown below so your media is recognizable to the UEFI based Surface Book.
One note - when you make your way down to the ISO image block and select your ISO file to use in creating the bootable media it will reset a couple of the settings you took care of earlier. My recommendation is that you select your ISO file first and then go back through all of the other settings to make sure they match the image above.
Once all of the settings are configured select the Start button and your USB drive will be ready within a few minutes. I found Rufus to be much faster than the old Windows 7 USB/DVD Download Tool and the newer Media Creation Tool.
Now that your USB media is ready for installing this version of Windows 10 to your new partition on Surface Book you just need to power down Surface Book and then insert the USB drive into one of the the devices USB ports.
Press the Volume Down button and then hit the Power button to start Surface Book. Once the Surface name appears on the screen you can release the Volume Down button and you should now boot from the USB flash drive.
Proceed with installing Windows 10 in that new partition by selecting the second option on the installation screen to create an install in a new partition. If you select upgrade then it will try to upgrade your current Windows 10 installation.
From this point the install continues like any other OS install and the dual boot menu will also be created so you can choose which version of Windows 10 to use at start up.
After you have the new install of Windows 10 up and running make sure you go back to File Explorer and turn BitLocker back on for that drive so it is protected.
Any questions on this process just ask away in the comments and I hope you find these tips useful.