Pretty sure I understand what you're wanting. There's a couple of things going on with motion detection and triggering as it relates to a single video file and whether it 'combines' multiple motion detections and subsequent triggering of that cam to put it all in a single file or separate.
First, regarding splitting long videos... At least one way you could do this is to limit the length of any single video recorded by a camera to a specific length of time using the 'Combine or cut video' setting and limiting the time to 20 mins... However, it sounds like in your situation, you're camera is continuously triggering during the lunch rush. This is fine and a new video will begin every 20 mins just like you want. However you should also note that if, for example, there was a lull in motion for 1 minutes, the recording would stop as there was no trigger for that time period, and upon the next motion trigger would continue to add to that video until it reached 20 minutes in length. This would have the net effect of a video that spans 21 minutes it total real time but is only 20 minutes in length with a 1 minute jump in time at the point where there was no motion.
Now, it's also worth noting that this behavior can be modified by adjusting the 'Break Time' setting under the Trigger tab. See screen shot. Let's say in your store example, you adjusted the Break Time setting to 70 seconds. This would essentially mean the trigger would stay 'on' for 70 second after the last motion that was detected and then would turn off. That means your video would capture continuously as long as there was no lull greater than 69 seconds otherwise the video would behave like I mentioned before. I would imagine that's not exactly what you were hoping for either as no motion in videos is just a waste of space in some ways.
The interesting challenge with your desire to not Combine videos is that as far as I'm aware it's not possible to limit a video to a time length without that setting being turned on which would mean all videos would be as long as the trigger was on plus the Break time amount. Others may know a way to do this...
But if you could live with the possible combine situation over lunch time you could do exactly what you want outside of the lunch time by creating two profiles in the main settings and call them something like 'slow' and 'lunch rush'. And you can control the schedule of when those profiles are active on the schedule tab in settings. Then inside of the camera settings, you can set the behavior for combining and a limit of 20 mins to the 'Lunch rush' profile and combining off for the 'slow' profile. That way, no video would ever be longer than 20 minutes during a normal day, unless there was actually motion for greater than 20 mins during the 'slow' profile. But hopefully this makes some sense. It helped me to just type it out. ha ha.
