You can have both versions of Xcode on a machine simultaneously.

Here’s what I did

1. I installed Xcode5 normally
2. I downloaded Xcode 4.6.3 from the Apple developer website and copied that into my Applications folder as Xcode_4_6.app
3. I downloaded the iPhone/iPad iOS 7.0.2 ipsw’s from the developer website and loaded them into the 4.6.3 Organizer (Devices -> Library -> Software Images).
4. In order to get Xcode 4.6.3 to see an iOS 7 device, you need to start Xcode 5, load a project so that it connects to the phone, and then exit Xcode 5 and start Xcode 4.6.3,  You periodically need to do this again (probably whenever you restart Xcode, but not every time you plug in the phone)

One annoyance is that both versions share Recent Projects and will automatically open projects that are open in the other one.  It’s usually best to only start one if the other is closed or doesn’t have any projects open.

I also read on StackOverflow that you can get Xcode 5 to create iOS6 apps by making a symbolic link from inside the Xcode 4.6.x app bundle to inside the Xcode 5 app bundle.