Here is the link to what Tablo Support told him: 2 tuner Tablo: Acess with 2nd device forces 1st device to change channel
What he is saying is that if you are recording a 720p show at the highest quality, it will use two tuners to transcode at 60 FPS. If only one tuner is available, it will only transcode at 30FPS. Since sports are the only events where 60FPS vs 30FPS is noticable, most of the time you would never know.