Hello SunsetRider! 👋
It's absolutely safe to store your Secret Key in your Private vault in 1Password. No one can access your 1Password items without both your account password and Secret Key so they would need to know your Secret Key to get to your Secret Key. A good analogy here is to think of a physical safe: you can store a copy of the key to the physical safe inside of the safe and someone would need to already have a key in order to open the safe and get the copy of the key.
Let me know if that makes sense and if you have any other questions. 🙂
-Dave