Can it be done?  I have tried to snap pictures while recording video and the app makes the shutter sound, but I don't think it's actually taking a picture.  Am I missing something here??

Not sure if it does, it's a fairly new feature on the Mavic and phantom, have you checked in your app to see if your flight recorded video and photo.

can not do this.  if we had RAW format you could take screen grabs from your videos and they would be good quality

Spark does not support to snap pictures while recording. You can try to grab a photo when you review the video at this moment.