Everyone has pretty much nailed it with the use of Flint n Steel but I might go into a bit more depth...
The first thing you have to remember when fighting a double team is that RUNNING IS OK and definitely necessary.
So first when you spot a team you'll want to make sure you have full hunger. Preferably eating the best food you can so your sprint doesn't die down too quickly. Once you've got their attention (either attacked them or them come at you) you want to run. Even if you are not low on hearts it is important to note that this can change very quickly with two swords hitting you.
Next you want to hopefully make sure you have at least one of the following... Flint and steel, bow and arrows (at least 15-20 arrows) or a fishing rod. So the basic idea is to SLOWLY deal damage to the player which is closest behind you until eventually he starts to run away or needs to eat. This can be done by turning around and firing an arrow every now and then or a couple flint and steel hits. A fishing rod may be a bit tricky but if you're skilled enough you can hit them without taking any damage.
While you are attempting to do this avoid running into any buildings, water (unless you have arrows and it's the only way), mountains, ladders and any confined spaces.
Once one has backed away it is a fair 1v1, hopefully you will have full hearts still (since you should be regening the entire time) but if not then continue the same tactic you used on the first guy until you think you are able to take him in a sword battle (as a last resort really).
Knowing traps on every map also works a treat. Sometime a weapon isn't even required if you know which traps to take people to and teams can easily be taken out.
Please remember also as I see it happen way too often... there is no need to risk your life to kill one of the people. If you get one of them low then RUN, attract the other guy because odds are the guy you got low won't be coming after you so you'll end up with a 1v1.
Hope this helped.. any questions please ask.
