Two Kinects can be used at the same time, but each of them will output its own, individual data. The sensors do not interact in any way. This is as the analysis algorithm does not account for multiple sensors - it's performed independently for each sensor's depth feed. Multiple sensors can be used, for example, by pointing them in different directions to capture more people, though. Do note, though, that the infrared sensors cause interference to each other if they are pointed in the same direction as the infrared grid they project will be mixed with the grid of other sensors.
Multiple Kinects have immense USB bandwidth requirements. You will probably need an PCI-e USB3 extension card for the second Kinect.