Weird behavior, indeed.
If your laptop had single USB controller, you would not be able to run 2 Kinects at all.
What is your laptop model? Modern Intel chipsets with built-in USB 3.0/2.0 controller have compatibility issues with 1st generation Kinects described here.
https://social.msdn.microsoft.com/Forums/en-US/b8ff7e7b-befe-426c-8eae-192a9aa3fa88/multiple-kinects-on-an-intel-usb-30-controller-are-failing?forum=kinectsdkYou may provide us a log from iPi Recorder (button in top-right corner) which probably contains some clues for the problem.