Davie's from the Lowlands, Alan's from the High. Davie knows so little about his family that he's not sure whether his uncle Ebenezer is older or younger than his father. Alan knows all of the Stewarts, their allies, and their rivals – and he's willing to put his life in danger carrying rents from exiled clan leader Ardshiel's estate back to Ardshiel in France. Davie is a true blue, loyal Whig: he supports the British king as the sovereign of Scotland. Alan is a committed Jacobite: he deserts the British army at the Battle of Prestonpans (in 1745) to fight on the Highland side to restore the Stuart family to the throne.
These guys are really, really different from one another. And yet, they complement one another perfectly: Davie can't speak Gaelic, so Alan translates for him. Davie can't fight with a sword, so Alan teaches him. And at the end, Alan is in desperate need of money, so Davie gives it to him. The friendship between these two men demonstrates that all of these surface differences – politics, religion, and so on – may be important enough to die or kill for, but you can still like your buddy, no matter what his opinion is on Parliamentary power.