Is the word "consensus" only used when a group of more than two people share an opinion? Or could you say that two people have a consensus? If so, is it common and acceptable to use this word in relation to two people?
Consensus noun [usually in singular] A general agreement: ‘A strong relationship means that it must depend on a consensus of needs and opinion.’ Synonyms: agreement, harmony, concord, like-mindedness, concurrence, consent, common consent, accord, unison, unity, unanimity, oneness, solidarity, concert - ODO
The wikipedia article on how their primary decision-making mechanism works illustrates an important aspect of consensus as opposed to simple agreement:
A consensus decision takes into account all of the proper concerns raised.
There are several ideas caught up in the term consensus in wider usage, a few of which are:
- agreement by everyone, or almost everyone in the group;
- the exercise of a group-based decision-making process, whether formal or informal; and
- full discussion, to the point that all alternative points of view have been heard and all objections have been satisfied.
The term almost everyone can be hard to pin down for a small group, and especially so for a group of two. Is only one out of two "almost everyone", or does it require unanimity? The sense in which consensus is typically used requires unanimity in that case.
The issue of having had a full discussion can be harder to express using some of the synonyms provided by ODO, though concord and concurrence do carry similar connotations. The phrase meeting of the minds, which wikipedia traces to contract law, has been adopted more broadly to express this kind of concurrence, particularly with small groups - even with groups of just two participants. Consider the following example (emphasis mine):
This is the first expenditure of a significant amount of money on each project, and may be the first time that the user and the designer come to a meeting of the minds as to just what is being designed. This key step has a success ratio that goes from 0 to 100, depending on the user, the designers, and the agency's sophistication in promoting a true meeting of the minds that is still consistent with the programmed intent of the project. - "Improving the Accuracy of Early Cost Estimates for Federal Construction Projects" "meeting of the minds", by Committee on Budget Estimating Techniques, Building Research Board, Commission on Engineering and Technical Systems, National Research Council
Finally, to answer your question: if we return to the example sentence at the start of this answer that I chose from ODO's list to accompany its definition of the term consensus, we find that there is precedent for the term to be used with a group of two. A note of caution: there remains some awkwardness relating to the numerical issue if consensus is not used carefully, but it can be used to emphasise the journey of reaching consensus.
The short answer
You can form a consensus with just 2 people but it isn't the norm.
The full answer
The definition of consensus is
a generally accepted opinion or decision among a group of people
Where a group of people is a number of people who are put together or considered as a unit.
Now the group of people would not normally be formed out of just 2 people but a study could be carried out by 2 suitably qualified people and if both reached the same conclusion it would be a consensus. If both cannot agree on the findings, there would not be a consensus and the result of the study would be inconclusive.
When talking about consensus, as indicated in the examples given in my link to the definition of consensus, it is usually in the context of a meeting of many people; such as a group of employees, a club committee, or a board of trustees/directors. There would be an agreement formed after discussion (maybe by vote if no clear consensus can be observed) and it is the majority decision which forms the consensus.
The more people who reach consensus, the stronger the consensus is.