Corporate pilot reported they were listening to; but not transmitting on; the correct MTJ CTAF frequency and flew too close to another arriving aircraft. The reporter stated a nearby airport using the same frequency causing radio congestion and interference also contributed to their confusion.

Narrative

Due to late cancellation of IFR and handoff to UNICOM frequency which included returning to ATC frequency to ensure the change to advisory was authorized I left the comm switch in the comm 1 (ATC freq) position. I was monitoring both comm 1 and 2 (UNICOM) and heard other operators and aircraft on same UNICOM. Some were in the MTJ pattern; and some were not. As we descended down final; I noticed a TCAS target (Aircraft Y) at our 9-10 o'clock and coaltitude. The TCAS range was still set to 15 miles; but the aircraft was outside the 2-mile inner ring. I continued to make recommended calls (albeit on comm 1) and monitor the traffic which eventually showed 600 ft. above and behind us at an unknown distance (Aircraft Y became at our 7 o'clock and then at our 6 to 5 o'clock). We landed without incident on runway 35 as we confirmed the runway clear of conflicts (visually and backed with TCAS) in keeping with our standard safety checks. After landing as I keyed the mic to announce off the runway; I noticed we were still on comm 1. I can only surmise that ATC couldn't hear our attempted UNICOM radio transmissions or perhaps we couldn't hear them due to terrain interference in the valley. When I announced off 35 on comm 2; the other aircraft (that had landed out of a VFR pattern) informed us that they had not been hearing us transmit and seemed surprised to see us crossing in front/below. They did not indicate anything additional. This report stands mostly as a reminder to PNF to not only announce the primary comms but also ensure that the switches align with what they believe to be true. Because there were no apparent; serious conflicts (Visually and with TCAS); I believed the position reports from the other aircraft (whose type I do not know) were advisory to me as I reported our positions (on the comm 1 freq). Another lesson learned is to have fields in close proximity deconflict UNICOM or CTAF frequencies as there were multiple calls that made even regular position reports difficult to include local operations and aircraft in a nearby pattern to MTJ; it is noteworthy that there were several transmissions in quick succession; and a few were possibly stepped on during simultaneous transmissions. An airport the size of Montrose should have its own CTAF or UNICOM to avoid this interference.