In Season 2, Marco Polo captured his father, who told him the plan of Crusaders, to divide and conquer the Mongols by letting the Mongols(Kublai & Kaido) fight each other, then the crusaders will wipe out the winning mongols too.
Kaido and Kublai was having a rift with each other during the kurultai, but they still respect each other. If Marco had told them about the Crusader's plan and their army who's coming to wipe off of Mongols, there would have been a chance that both of them would join forces to defeat the Crusaders.
So, maybe Marco didn't trust his father's words. But after seeing the Crusader's Army Marching towards Karakoram, he went back to save Kublai, who was being choked by Kaido at that time. He knew Kublai's would need his neighbor's alliance to defeat the chainmail-Iron/steel-armour-plated Crusader's army who is marching towards karakoram. He could have knocked off Kaido instead of stabbing him in the back. He could have then told Kaido and Kublai about the plan of Crusader's then.