You can still build a test your app on 2 and then submit using Xcode 6. I run beta iOS updates and then build production apps with the old Xcode - you don't need a device plugged in to do this. However you're risking missing bugs as your test OS doesn't match your build OS. As for switching between watchOS 1 and 2 - downgrades are not supported or reccomended.
Topic:
App & System Services
SubTopic:
Core OS
Tags: