Setting up and choosing the proper MIDI channels is how you can separate these instruments.
In Options/Preferences(OS X) > External Devices. Set up each New Keyboard to use a particular MIDI channel, not all channels.
Then, make sure each device is set to transmit on the same MIDI channel.
When you create a new song, add your instruments and assign each track to the respective device.
We also encourage you to create a thread about this on the Presonus user forum. https://forums.presonus.com/viewforum.php?f=108 This will allow for back and forth discussions about an issue or question and more points of view as to possible different approaches or workflows.