It can be used. You just need to find the file for the 8.x sdk that you want to test on in Xcode 6.x and copy it over to the corresponding Xcode 7 beta folder. You can find the file in this path.
Path: /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport
Xcode will suggest that you fix an error or two, but it should work. Note that when I did it there was still an error and some warnings, and I clicked run anyways and it worked regardless
You may need to click "other" and add "iphoneos8.x" (x is your version number)and if you have it added to the correct location it will work
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