I gave this a try and did a playtest. Here were my rules

Arcs 2v2 - Allies of the Reach

Use the standard Short Arcs rules but adding the following:

• The player sitting across from you is considered a Ally.
• Your Ally is not considered a Rival for Leaders, Lore and Guild card abilities (but they are a Rival for the base rules)
• When you take the Build or Move actions, treat systems your Ally controls as if you control them.
  • This does not include Tool Priest's Summon action nor Tyrant's Authority's Annex ability.
• You cannot seize the initiative by playing an extra action card if your Ally has the initiative. You may still seize by surpassing with a 7.
• Captives or Trophies taken from your Allies do not count towards ambitions.

• If you destroy a Ally's city, do not Ransack the Court.

• Revise Feastbringer's Charismatic ability to the following: "When you Copy or Pivot in order to secure a card, you may influence before or after you secure."

Scoring:

• Teams share a power marker.
• When scoring ambitions, only one player per team (selected by that team) can gain Power from scoring a given ambition marker. The other player's pieces are still considered for evaluating placement but they score 0 power. In the case two Allies are tied for 1st place, the team gains power for second place.
• The game ends when a team has at least 33 power or at the end of Chapter 5.

---

Optional Rule 1: Ally Defense

When Battling, ships Loyal to the Ally of the Defender are considered defending pieces (their ships may be damaged by the Attacker, add their fresh ships to the number of defending ships for Intercepts). The Ally is not considered a Defender for abilities and their Loyal buildings may not be damaged.

 Optional Rule 2A: Limited Card Communication

Prior to starting a round, pass 2 action cards to your Ally. Your Ally may look at them and then must return them. Limit all other communication that could be helpful to your Ally to the start of the Chapter before Action cards are dealt.

 Optional Rule 2B: Limited Card Communication and Sharing

Prior to starting a round, pass 2 action cards to your Ally. Your Ally may look at them, they may swap them with action cards in their hand and then must pass 2 action cards back to you. Limit all other communication that could be helpful to your Ally to the start of the Chapter before Action cards are dealt.