The first that comes to my mind are Rainbow Six Rogue Spear: Black Thorn , Battlefield 2, Counter-Strike: Source for online and PixelJunk Monsters for offline. All four games with my cousin. BF2 and PJM was probably the best experiences.
In Black Thorn I always took the higher areas while he took the lower areas, but we worked on the same side/part of the map if one needed cover or help. Just a shame the game lagged so much. :(
In BF2 I played as a medic and my cousin as an ammo-guy(support?) and we held the posts/flags pretty much alone for complete game rounds. It was so much fun until EA/DICE decided to patch the game and remove the "bugs" that everybody liked.
In CS: Source we always worked together as a two-man-team protecting each other. I mostly flanked in bigger areas(like B-doors and A-long in Dust2) and he gave me cover, but he played better on narrow areas(like catwalk in Dust2), so he went in first and I gave him cover.
In PJM we always had different tactics for every map. One would collect the gold from the trees while the other one would start building up defense. And then we held two sides each, for some maps it meant that I kept the "nest" safe and he kept the corners and "entrance" safe. Or we had one side each to protect in maps with multiple entrances.
This thread really brought up some good memories for me. Thanks. :)








