There is as much evidence of Montagna and Desjardins working together as there is of their involvement in the plot to take over in Montreal. I believe that Montagna acted as an ambassador for the rival factions and that he played a key role in the plot to overtrow Nick Rizzuto. When Nick Rizzuto learned that Montagna would be deported to Montreal he send back word saying that Montagna was a light-weight flunky and that he wouldn't take orders from him. The question then arises, if Nick Rizzuto was a boss of his own, why would he say such a thing? It implies that he would've been prepared to take orders from someone from New York who he did held in high regard. And that also implies that the Rizzutos still answered to New York. In any case, Rizzuto's remark on Montagna seems to have set in motion the plot to depose him. I believe that if Montagna hadn't set foot in Montreal the murder of Nick Rizzuto wouldn't have happened. I think that Montagna was the missing link between Montreal, New York and Toronto and that he brought the rival factions together to initate the take-over.

Afterwards, Montagna probably didn't want to share power and regarded Desjardins as an obstacle and tried to take him out. He failed and Desjardins was left with little choice other than taking Montagna out before he would be hit again. Di Maulo likely supported Desjardins and this, as well as him betraying the Rizzuto family, came to haunt him last year. I think things may have looked entirely different had Montagna not jumped through a window and ended up on the bank of the river.