It is possible to combine the two devices, feeding the concentrated oxygenated air from an oxygen concentrator into the inlet of the CPAP. I suspect that the oxygen boost would compensate for the much more shallow breathing during sleep.  There must be some research that deals with this issue.  The use of oxygen concentrators are routinely prescribed during sleep for the elderly who do not have an obstructed airway that would require the CPAP forced air respirator.  The issue here is how best to optimize oxygenation during sleep.