To me, it depends on the purpose of your app/site etc. If the goal is to achive something by yourself, testing should also be done with one participant at a time. Using more that one participant coorperating will give very different results, as theoretically with 2 participants only 50% need to know how something is done to ultimately get a 100% score.
Even worse: because they are cooperating they can feed of each others ideas and observations, both achieving more that they would indivually do. For example: examine this scenario:
Person 1 (p1): I have no idea what to do
Person 2 (p2): What if you click here?
p1: Hey that works!
p2: All right now I have no idea
p1: What is you slide this to the right?
p2: Yes that is the solution!
In this case, individually, neither would complete the test, as p1 would be stuck at the first step, while p2 would be stuck at the second step. That could lead to the conclusion that the product needs a lot of work. However if you just look at the results of the collaboration, your instinct would say that everything is working perfectly as they both achived the goal!