Firstly ensure that you selected the same Google account when you installed Mabui on device 1 and device 2. If you didn't, you will need to uninstall Mabui from device 2, reinstall from the Play store and select the matching Google account.
On device 2 go to "Install apps" from the main screen and select the "Private" option. You will see a list of all your private apps - just tap an app to install it.
You can now use your app on both devices.