Windows 8.1 Preview won’t be delivered to Windows 8 users exclusively via the Windows Store. Microsoft will also release standalone ISOs files available on its official website.
Specifically designed to help you set up a dual-boot configuration, these ISOs will allow users to install the operating system on a different partition.
“If you want to install as a dual boot, we do NOT recommend downloading the preview from Windows Store. Installing from the Store will upgrade the existing Windows 8 OS on that partition to Windows 8.1 Preview," Microsoft explains in some leaked documents.
"You will not be given an opportunity to select another partition, so you would need to already have a dual boot system installed in order to install Windows 8.1 Preview as a dual boot through Windows Store.”
Here's a screenshot of the leaked document:
Source: Leaked documents