Apple has just released MacOS Sierra 10.12 in Beta version. You now can install Sierra - Dual Boot El Capitan.
Step 1: Make Bootable MacOS Sierra 10.12 Beta USB Installer Drive
We share a very specific instruction in another topic: http://applehint.com/t/instruction-to-download-and-make-bootable-macos-sierra-10-12-usb-installer-drive/754/
Step 2: Create a MacOS Sierra Partition
Open Disk Utility, the app is found in /Applications/Utilities/
Select your hard drive from the left menu list
Click the “Partition” button, then click the [+] plus button to create a new partition
Name the new partition you are going to install MacOS Sierra. Whatever name you want, but you must assign the partition a sufficient space (20Gb minimum). Click on Apply to finish and to create the new partition on the drive.
Install MacOS Sierra to the New Partition
Plug the Bootable MacOS Sierra 10.12 Beta USB Installer Drive into your Mac. Restart your Mac, Immediately hold down the Option keys after you hear the startup sound, you then will be able to select the USB to boot and start installing MacOS Sierra 10.12 into the created Partition