The easiest way is to use a USB cable connection as most of arturia Midi controller are USB powered :
For this example we will use Cubase 7.02, and 2 Analog Experience the Laboratory.
1/ Add a new track:
==> Project --> add a track / instrument track
2/ Chose Analog Laboratory Software
3/ Under the drop Down where is written "All Midi Input" :
==> Choose "Laboratory"
4/ Replicate these three steps to create a second track.
Once the time to choose the second MIDI Input, a second Laboratory will appear.
Unfortunately Cubase does not distinguish between the two keyboard, Like Ableton, or Logic, so you have to manually pickup the correct Laboratory.
To make sure you can play them both at the same time, make sure the record button is correctly ticked.
Let me know.