Foe Yay/Tabletop Games

Examples of in  include:

Card Games

 * Chandra and Jace from Magic: The Gathering are implied to have this sort of relationship, judging the flavor text of many of their respective cards. The tie-in works seem to confirm this: they often unite to defeat a common foe and even save each other's lives from time to time, but still chase each other around a lot. In honor of this, Wizards has released a pair of preconstructed decks reflecting the two.

Tabletop RPG

 * This trope actually killed one of the vestiges in the Dungeons & Dragons Tome of Magic (3.5 ed.) -- Agares, Truth Betrayed. He deeply respected a recurring adversary, so suspecting a Foe Yay taking place, one of his lieutenants delivered a fake challenge to a duel with that adversary and then murdered Agares.
 * In the Edition War series on the D&D Forums there are at least two cases of foe yay (and technically there's Puppet/Shade but Puppet is Shade's mom so we don't like talking about that).
 * Forgotten Realms has their share, the most famous being the archwizard known as Lord Shadow (Netherese who discovered the Demiplane of Shadow) and, one of the most talented among the hundreds of assassins who hunted him. They tried to kill each other, then saved each other, then decided they preferred the latter. And eventually "vanished" together when attempts on his life became too tiresome.
 * How about Artemis Entreri's obsession with Drizzt Do'Urden?
 * Another D&D example occurred in the Mystara setting with the Immortals (D&D's functional equivalent of AD&D's gods) Ixion and Nyx. Ixion was the reigning Hierarch of the Sphere of Energy, and the Immortal of the sun, light, and fire, while Nyx was a Hierarch of the Sphere of Entropy, and the Immortal of night, darkness, and the undead. They constantly opposed one another, but got along very well in their "off hours," and were clearly attracted to one another. The fluff never made it clear whether they had ever consummated their flirtation.
 * White Wolf's Exalted setting lends itself well to this trope, since two of the "bad guy" exalt types- Abyssals and Infernals- were once Solars in the First Age, and may have been close friends or lovers to Lunars, other Solars, Sidereals, etc.
 * Hell, plain old Solars and Lunars can have this. The nature of the Solar Bond means that Lunars are often lovers to Solars, but can just as often be best friends, boon companions... or rivals who challenge them on every level. And those categories can easily shift.
 * Official characters Harmonious Jade (Solar) and Disciple of the Seven Forbidden Wisdoms (Abyssal), former First Age lovers, seem to have a bit of this going down.
 * Implied to be canon between Deathlords First And Forsaken Lion and Princess Magnificent With Lips Of Black Coral (at least on Lion's side).
 * This is canon for the Gentry in Changeling: The Lost. Their home dimension, Arcadia, is made of such swirling chaos that the only way for the Gentry to maintain power and coherency there is to make war on one another. So, a Gentry's best friend among the fae is the one who will try their hardest to stab them in the back for tales of glory.
 * And this is sometimes a literal example. The chapter fiction is a romance between characters controlled by opposing True Fae.

War Games

 * The Dark Eldar of Warhammer 40,000 are quite probably the masters of this trope, as while they fight solely to increase the amount of slaves on which they can inflict horrific tortures, their attitude and behaviour towards this endeavour is distinctly sexual in nature (to the point where they have become Memetic Molesters in some parts of the fandom) and especially in the context of the World Half Empty setting its almost as if that's how they normally get dates. For a specific example, the Eldar after action report for the Dark Eldar campaign in Dawn of War: Soulstorm notes that Archon Tahril was disappointed that Farseer Caerys escaped capture, as he wanted her to be his "personal plaything". Seriously.
 * What's squickier is that there is fanfiction of that.