I've got two sets $A$ and $B$, both with $n=2$ elements / measurements. The variance of both can be assumed to be similar / depend on the same measurement error-function. I need to compare (with a t-test) a third set $C$ (also $n=2$) of independent measures to the set $D$. $D$ should reflect the sum of the true value (without measurement errors) of $A$ and $B$ tries to measure, but also have a variation that could be expected of $D$ based on the variance of $A$ and $B$.
My question is simply: How to construct $D$?