Use the same AppleID, and then turn on the automatic download function. Take the iPhone7 mobile phone as an example, and the setting method is as follows:

First click the "Settings" icon on the mobile phone desktop, as shown in the figure.

Second, after entering the phone setting interface, open your own AppleID bar to enter, as shown in the figure.

Third, after entering, select the options in the red box below, as shown in the figure.

Finally, turn on the switches of "Application", "Book Audio Book" and "Update", as shown in the figure.