I purchased the IK Multimedia Hammond Organ plug-in for my Studio One Artist 5 recording software and the user manual seems lacking in instructions regarding MIDI connections/menu choices for getting two keyboards to access/activate the two virtual keyboards of the plug-in.
At present, my Yamaha keyboard MIDI out goes to my Roland MIDI in and the MIDI out of the Roland goes to the interface box.
I'm not real MIDI savvy and need some basic instruction so I can use both of my keyboards to activate the two virtual keyboards of the Hammond.
I do not know what device is meant with "Roland", but I guess that that data at its input is not passed through to its output.
You then need a USB/MIDI interface with two inputs, or two interfaces.
As Clemens Ladisch noted, one likely problem is that the messages coming in to the Roland MIDI IN port are not getting sent out on the Roland MIDI OUT port. Some Roland devices have an option called "Soft MIDI Thru", see if you can find an option like that and enable it. It will pass a copy of MIDI messages that come into the MIDI IN port out to the MIDI OUT port.