Is Risk: Legacy playable two-player?
It is a three to five player game according to the rules because the negotiation dynamics are integral to the rules. With two players there can be really no good negotiation and the game loses an essential element. Tweaking the game would definitely be needed, especially in regards to some items found in secret passages.

So in the end, since you would play just as long as the game doesn't crash and burn, I would play with one of the 2 player variants that can be found online (basically both Players get 2 factions).