Unfortunately, even with Elk sensors, the M1 cannot perform direct comparisons between mutliple sensor values. You will have to write rules for actions based upon sensor values beiing "greater than" or 'less than". Depending on what you intend to do, this can take up a lot of rule space. Don't forget that if you use the "and" function you are using up an additional rule space as well.
In my new house, I have the M1 and a TRS 40 as well, but my TRS is controlled by the HAL software program, so neither controller can pass data back and forth, and the Hal cannot interpret sinsor inputs directly either. I have wiring in place for both an outside temp sensor and one up at the vaulted ceiling. As of now I have not yet integrated them into a complete energy management program. (I am still finishing the house for occupancy and have higher priorities for now).
If energy management is your goal here is where I left off planning:
I am thinking along the lines of having Elk detect the TRS output for heat demand (RH wire) as a contact closure (or possibly a voltage) in parallel with a relay output and then activate a loop that will check the value of the ceiling sensor. If it is 5 defrees warmer than the TRS, the Elk will activate the ceiling fans for say 2-5 minutes to blow the trapped heat back down before passing the RH voltage through the relay. If the heat demand goes away then the fans will shut back off and the output relay will never get energized. If the heat demand remains, the fans will remain on until the heat demand from the TRS is removed. Outside temp could also be integrated into the A/C demand cycle with a bypass damper.
I was discussing this with Martin over at AO and he has experience with multiple temp sensors and the Elk. Maybe he has some answers, or can direct you to another thread.