A physical token only authenticates itself as "something you have" if there's no way to extract the key from it. In practice non-hardcore deployments usually have a backup procedure but in principle, if you want multiple tokens, they should have separate keys. What you're asking in simplest form involves storing the key on a server where it can potentially spill in a server breach or the like. If the key protects something very valuable, that can be dangerous. If it's for your old Reddit account, you might decide to do it anyway.