Connect L out of keyboard to L input on interface.
Connect R out of keyboard to R input (2nd one) on interface.

If the interface supports it, pan the L&R to 100% L&L, otherwise you will record both outputs to both channels/tracks.

Then in your software, mix the output as desired.
The key is to get both the L&R outputs of the keyboard into the computer .. then mix them after they get routed as such.

Your unit has a Stereo/MONO switch, which may come into play.

I do not work here, but the benefits are still awesome
Make your sound your own!