If you are in a secret alliance, perhaps taking two continents that are not adjacent is a better plan since it will be harder to tell if you are aligned or not. It may become obvious that you are not attacking each other across the borders (of if you do attack to trick people, you don;t attack vigorously enough) and you can use pincer attacks on your opponents. I think the best two continents for this strategy is actually Africa and North America. I don't normally like Africa as a starting continent but if you know you have help from North America, you can relieve the pressure that South America and Europe are applying. The key is to have your Alaskan border secure in North America either through public diplomacy or because Asian territories are more spartan than your mega-stacked Alaska or Kamchatka and also the Middle East for the African player. Asia/Australia will be the lynchpin in this deal.
Be sure to apply even pressure to both Europe and South America. You need to keep them worrying about keeping their own continents so they can't shift their focus onto taking out your continents. This 1-2 punch when applied evenly through out the game will eventually lead to success. If North America goes first and takes out a couple of European territories, Africa can go into South America. They can switch next turn to keep their opponents off balance or even split their forces both ways. Be sure the attack method you choose doesn't become an obvious dance, or your opponents will be able to tell easily.
When I play Risk, my friends and I are ALWAYS suspicious of secret alliances and we often engage in backroom diplomacy (you know, one goes to the bathroom, the other gets a beer in the fridge, as they pass each other a message is dropped, etc.) so we are actively on the lookout. So many of the diplomacy startegies in the website apply directly to our games since we remember each other's tendencies and whether we backstabbed or engaged in questionable conduct (such as a secret alliance).