It is also worth to mention the experimeters feelings which reported more stress when demanding compliance from two subjects than when demanding it only from one subject.

Number of reactions
Alone subject (N=31) Two subjects (N=31)
Comply Not Comply Not
Two men (man) 7 9 3 7
Two women (woman) 10 5 1 7
Man and woman - - 6 7
Total 17 14 10 21

Table 1.
Figure 1: The number compliant|non-compliant reactions in the
one-target(N=31) and two-targets(N=31) conditions.

DISCUSSION:
However simple this experiment may to seem, it shows significant results. Maybe it could be done as a more complex one: variable number of experimenters would demand compliance from 1, 2, 3, 4-member group. The subjects should be chosen in more precise way to avoid differences in their mood etc. It should be also controled the sex of the experimenters and the subjects, or whether the pair in the experimental condition is mixed or not.
References:
[1] Low-Ball Procedure for Producing Compliance: Commitment then Cost, In Journal of Personality and
Social Psychology 1978, Vol.36, No.5
[2].