Let's start with an obvious one. Twice as many creatures showing means twice as much life. Tons of decks have pairs of these, and I'd be shocked to see any in sideboards this weekend unless Forests were absent.
This one is a lot trickier. Often hovering beneath the playability line in one-on-one, Frozen Aether literally doubles in power in 2HG for the same mana cost, essentially tapping two opposing lands a turn, turning off the haste of tons of suspend creatures, and making opponents' blocking especially awkward. The fact that Frozen Aether usually comes out just before most creatures come out of suspend, basically suspending them a full extra turn, makes it especially nasty. A teammate's creatures can help keep you from falling behind in board tempo while playing the Aether out.
In one-on-one play, this common Illusion is used to making 2-4 of your creatures unblockable when it unsuspends, giving you the opportunity to deal 4-8 damage at the risk of a counterattack just as heavy. If the board stalls out, he can (slowly) offer a final breakthrough. In 2HG, however, the Oddity makes all creatures unblockable, not just your own, usually sending 4-8 creatures through unblockable for about 8-16 damage. If suspended on turn four alongside another two-drop, the Oddity can even send through more creatures than that, soon dealing the 22 or so damage your team needs to win. If the Oddity gets destroyed, the rest of your team is still unblockable. Since two heads' worth of unblockable creatures will often be lethal, the opposing team never gets the opportunity to counterattack and even the score.
Getting the most 2HG attention on the Internet, storm cards in Limited can go from "yeah, okay" to "wait what??" when you move to two-on-two games. Two players suspend cards to set up the storm turn, then when they resolve, Player A plays a bunch more cheap spells, then Player B drops Empty the Warrens, Grapeshot, Ignite Memories, or Volcanic Awakening with 3-5 copies.
Forestwalkers, swampwalkers, islandwalkers, and mountainwalkers provide critical evasion in 2HG, since your opponents can be counted on to have four or five types of basic land in play. By the rules of 2HG, if either opponent controls the appropriate land type, your landwalkers are completely unblockable to both opponents' creatures. Likewise, "islandhomish" creatures like Bog Serpent, Slipstream Serpent, and Pirate Ship can almost always attack in 2HG, making them way more powerful than you're used to them being in one-on-one.
I saw one enterprising team building low-spell-count decks around this interesting Future Sightrare, who says "Whenever a player plays a noncreature spell, each of that player's opponents may draw a card." In 2HG, there are twice as many opponents playing noncreature spells, so the number of times the Storyteller triggers will double. Furthermore, since it says "each of that player's opponents may a draw card," every time Storyteller triggers in 2HG, your team will draw twice as many cards as you would draw in one-on-one. With twice as many triggers and your team drawing twice as many cards per trigger, your team draws quadruple the number of cards in 2HG from Storyteller that you would in one-on-one.
These guys activation costs and echo costs make them pretty clunky for one-one-one play considering that you sometimes won't have any targets for them, whether those targets be one-toughness creatures or fliers. In 2HG, with 4-5 colors played by your opponents and twice as many creatures on the board, you're way more likely to have targets for these guys, and players are valuing the Channeler more highly.
Just like the Channeler, this guy needs 1-toughness targets, and 2HG offers up more opportunities to spot them. But more than the Channeler, the Shambler also scales the impact of its tremor effect with the number of those targets in play. Also one of the few ways to stop Empty the Warrens.
Ok, I really mean Dawn Charm, which also offers a fog effect. Since a lot of 2HG power plays involve making huge squads unblockable, random fog effects not only keep you alive, but let you swing back past your opponents' tapped masses to score your own win. I did see a team talking about the actual Darkness, but it didn't make it into their decks.
These examples lead to broader categories of cards that change value in 2HG you can think about when playing alongside your own second head:
Cards that scale with an increasing number of creatures/players
Cards that get better when your opponents have a narrow kind of permanent in play
Cards that enable or stop massive alpha strikes
Cards that you can build both your decks around and opponents won't
Look to take advantage of these categories of cards that get better in your next 2HG game!