implies that the GTR shown is only for the tank you're breathing from. In a sidemount setup, I would want the GTR to show the time I have left with the total gas left in both tanks.
My understanding is that the GTR shown in SM mode is based on total gas left in both tanks with the same mixture. The information is confusing since there are different computers with different firmware versions. The iX3M and the new watch computer with the latest firmware appear to show the total GTR.