Here's how you can get "Harrowmont's Ally" and "Bhelen's Ally" in one playthrough.
Note: I'm paranoid about closing storylines off too early, so I usually delay picking a side until the very last minute. So there might be a quicker way to do this, but this method definitely gives you a decision point where you'll only have to replay about two minutes of story in order to get both achievements.
First of all, the setup: I basically played both sides as far as gaining their trust. I accepted the initial tasks from both houses, got both of Harrowmont's champions to fight again, and turned both of Harrowmont's allies into enemies for Bhelen. At that point Harrowmont's emissary got mad but I agreed to play double agent for him, and cleaned out dust town.
Now you should be in a position where you're still begrudgingly trusted by both factions, and they'll both try to get you to go find Branka in order to get her endorsement. You'll also be finding out that Bhelen's pretty much a tool, but that's beside the point.
Play through the entire Anvil of the Void storyline. Whether or not you choose Caradin or Branka, you'll be given the opportunity to have a crown made so that you can basically choose the next king. After you get the crown, SAVE the game. This is your decision point. Leaving the anvil room you'll be given the choice to fast travel directly to the council chamber, where a cutscene immediately plays.
For "Harrowmont's Ally" declare that Branka or Caradin chose Harrowmont to be the next king. He'll thank you, Bhelen will reveal himself as a true douche, and you'll pretty much wipe the floor with him. After Harrowmont thanks you and gives you his pledge of aid, the achievement will unlock.
If you haven't gotten "Bhelen's Ally" at this point, just reload from your save and follow the instructions in that achievement guide.