Q. If there are two people pulling on a rope with force F, what is the tension?

A. Many people would instinctively answer this with 2F, adding the two forces. However, to correctly answer this question, one needs to carefully consider Newton's Laws - specifically the third law.

As each person pulls on the rope, the tension in the rope pulls back on the person. The pulling and the tension pulling back are a "third law action-reaction pair" and thus they must be equal and opposite., that is,the tension is equal to F.

from this, we can see that the second person is "useless" - the same tension can be achieved by having one person pull ont he rope after it has been attached to an immobile object. If twice F is required, then you need to place both people on the same end. It is easy to see that this would result in a tension of 2F.