I have played something like..25 games among friends, and like...5 of those have been 1v1s. we have never had a 2v1, but hell, we have had 1v1x5 and thats just gay and insane
There are rules for firing into melee against 2 enemies fighting a combat (on a 1,2 or 3, it hits the other squad, on a 4, 5, or 6, it hits the intended targetted squad.)
Pretty much we always make points fair, but i see a lot of teaming and ganging up, especially coz Im a bit of a ruthless player. Not yesterday, my blood angel friend skipped his entire turn because he didnt want to attack the 3rd player or myself, for fear of offending / eating the full force of their army. So it was my turn, and instead of being gay and doing that I...uh..unloaded on the only blood angels melta squad so he couldnt kill my tanks anymore (small points game) and then manticored a huge bunch of tyranids. The players were slightly crippled, but the game never finished.... ( i never lost a single guardsmen during that game, i really had all the cards, i hate unfair games.)
I prefer 1v1 , but my friends dont because I usually have the money to buy a good army, And dakka to help me with that
2v2 is fine, play it as if each team is one team, share points between the teams ( as in 750 per person so its 1500 versus 1500) And say i need a chimera and my team mate has 55 points left with nothing to buy, hed let me have it, for the good of the team.
free for alls always end in tears, 1v1s excuse you from favourtism / ganging up.