This question is asked frequently and I was presented with the same decision recently.  I personally believe 2 separate cases are better than 1 large model. This patient came in today and needed an e.max full coverage on #31 and an onlay on #14 which was done in Lava Ultimate.  This case took a little less than 2 hours and it was because I was on the phone for 15 minutes.  

The appointment went this way. I got both teeth numb. While the patient was getting numb, I imaged the upper arch and lower arch teeth that were not included in the prep image.  I then prepped #31 and imaged it and the buccal bite. I quickly designed the crown and started it milling.  As soon as #31 started milling, I prepped #14, imaged and designed it. This was done before #31 was even finished milling.  Then the phone rang and I got hung up for a few extra minutes. My assistant stained and glazed #31 and I tried in #14 which fit awesome.  I polished it and bonded it. We then tried in #31, bonded it in and cleaned up.  Took 2 final pa's and the patient was off to the races.  I think trying to do 1 large model would take way more time to image. I believe the fastest way to do CERECs is to leverage prep time with milling time of the other unit.