I think you may be confusing Shearwater's calculations of gas on-and-off gassing with its calculations of Gas Time Remaining. The former depends on which gas mixtures are activated, and has nothing to do with AI; the latter uses only gas pressure in either T1 or T2 (you choose, the computer does not) and has nothing to do with which gases are activated.Thinking it out loud here.
I understand that shearwater algorithms don't take more than two into account for dive planning.
They take activated cylinders only.
Therefore you could have four tanks on you with only a single t1 transmitter and the algorithm would work. Dive planning algorithm.
It appears that they've programmed two transmitters as such:
T1: planned cylinders
Can be two and more
T2: backup cylinder
Your bail out - usually the 40% and above ean tank