I'm using Xcode 7.3.1, which supports up to iOS 9.3.1.
Now I'd like to check my app on a device on a relatively current version of iOS (10.x), so I added some OS Support folders (using Xcode 7.3.1 with iOS 10 support ), which seems to have worked well.
Now I need the latest OS support folder (i.e. 10.2.1(14D7)). I have checked other StackOverflow answers, but I haven't found one that helps.
On
Is there any reason that you need to stick to Xcode 7? If you want to support older iOS versions you can download different simulator in Xcode 8. And also, you need to build in latest Xcode to support latest iOS. Xcode 7 doesn't have iOS 10 SDK.
As @MacMark said, you can keep both Xcode 7 and 8 in your mac but I don't see any benefit for doing so.
Just use Xcode 8. It supports iOS 9 and the latest iOS 10. If you need to keep Xcode 7 in order to run on older iOS versions you can just rename your Xcode to Xcode7 and install Xcode 8 next to it. You can run both of them but it would be wise to do it not at the same time.