Is there a way to have 2 devices on a single Google account
Google Play is only for Android apps. Android apps do not work on the Chromebook, which is why you only see the tablet listed in the Play Store. The Chromebook is basically the Chrome Web browser wrapped in a keyboard and screen and that is it. 

You can check the Chrome Web Store for an app for her bank, but even if there is one, it is likely just going to be a shortcut that loads the bank's website. If there isn't an app for it in the Chrome Web Store, just bookmark it for her. 

And to answer the overall question, you can have as many Android devices tied to the Google account as you want and it isn't a problem having a paid app on more than one device, as long as it is signed in to that Google account.