Control an Android app from another app using Bluetooth

We have written an app that uses a Bluetooth credit card reader to process payments for taxi drivers. The app works satisfactorily, except that it requires that the driver ask the passenger for the card to be scanned or for the driver to hand the mobile phone to the passenger, along with the reader. Neither of these options is optimal. Instead, we would like to provide a second tablet, mounted in the back seat, but controlled by the driver app, as shown in the attached image. This is the overall scenario: When the driver is ready to accept payment he launches the payment app on his phone, enters the amount, and transmits this data to the back seat phone. This passenger app wakes up, displays the amount due, allows for entering of a tip and instructs the passenger to scan the card. The card reader transmits the scan, along with the total, to the driver companion app, which relays the transaction to the gateway and waits for approval. When the approval comes back the passenger tablet displays the results and asks for a receipt preference: email or printed. There are two communication channels going on simultaneously: one between the driver app and the gateway, over an Internet connection, the other between the driver and the back seat tablet. Your job would be to modify our app to implement this second channel. We think that the passenger tablet must be ON all the time and running the slave app in the foreground, waiting for input from the driver.