How many keyboards in total?
If the total number is 2, including the Motif, the Motif will connect both keyboards, independently, to your computer. The Motif is actually a USB MIDI Hub. So if you connect the Motif via USB, the second synth can be connected to the Motif's MIDI I/O ports and accessed via any/all of the 16 MIDI channels available on those ports without impacting the USB MIDI connection to the Motif sound engine.
I've utilized that approach from time to time with a Motif ES6 and an S90ES. Works perfectly!
The Motif series is not the only controller to offer this functionality; I've got Korg and M-Audio drum pads that do the same thing. As does my 828ES audio interface.