This is a simple list of cards and a jumping off point for a friend of mine to make a pair of Two-Headed Giant Commander decks for him and his wife. It only lists cards that I believe are the best color combos that involve helping your two-headed partner, such as cards with "Partner with..." as well as cards that target another player that are beneficial to both of you, cards like Scheming Symmetry for example. The other deck is White and Blue.
This is by no means a complete or comprehensive list of every card that would be good for this deck, I don't even know what common oracle text snipets to type into srcyfall's oracle search for cards that would be helpful or synergistic with these decks.
Another way to go with these decks is to go into voting mechanics like Tyrant's Choice or Capital Punishment because you'd have two votes going for you and two against. I'm also including Monarch cards because why not.
MONARCH RULES CLARIFICATION FOR TWO-HEADED GIANT:
There can only be one Monarch in a game, when the player who is the Monarch is damaged by both players on the opposing team, the Monarch chooses which of those two players is the Monarch, this is because the Monarch mechanic triggers twice for each opponent, and the current monarch decides how those triggers are ordered on the stack. If only one player attacked your team, that player becomes monarch.
I've Put Tempting offer cards in the maybe board of this deck, as I thought they would say 'Each Player' does a thing, but instead it says 'Each Opponent' does the thing, so it doesn't help your partner out at all. But tempting offer cards are still plenty tempting outside of this fact so... idk if it should go in or not.
For a reference of all cards with "Partner with..." this link will list them all out, including cards that are green. I couldn't get a hyperlink to work properly for this link so you will have to copy & paste it.
https://scryfall.com/search?q=o%3A"partner+with"ℴ=color
