Using multiple devices with the same cloud accounts

I bought a new phone and everything is now up and running, and the old one has been disconnected from the Internet since I set up the new one. Now I'm considering using it as a backup phone for situations where I could lose or break down the new one. Is it ok to connect both phones to the Internet at the same time or will that mess things up? I'm thinking of problems like different versions of an application writing configuration info or metadata in different formats, and messing up one another.