Question: how many data points are enough for standard deviation?
Answer: You propose to have 2 data points, therefore requiring use of Student t with 1 degree of freedom. 95% 2-sided error bars for n = 2 data points require a multiplicative factor of 12.71 on the sample standard deviation, not the familiar factor of 1.96 based on the Normal (Student t with ∞ degrees of freedom).