The game should transfer your content automatically if you sign with the same Google Account on both devices. If you are having trouble with the sync function, try to follow these steps.
Before you start
Make sure you have Google Play Games installed and updated on both devices.
Manual sync from one device to another
Check that you're logged in with the same account on both devices
- Open Google Play Games app.
- At the top, tap Morethen Settings
- You will see your active account.
- If they don't match, switch to the account where you have progressed in your empty device.
- Under Your Data you'll see Change account for games
- Select Kingdom Rush Vengeance, Change, and the desired Account
In the game you already have content, tap the icon in the bottom left to enable cloud save.
Next, activate the same game settings in the second device (the one that is blank).
Let us know the steps you've already taken!