I'm having trouble figuring this out, but I'm pretty sure it is only a few lines for someone who understands this a little better:

"Suppose that $\displaystyle T_1$ is sufficient and $\displaystyle T_2$ is minimal sufficient. Let $\displaystyle U$ be an unbiased estimator of $\displaystyle \theta$ and define $\displaystyle U_1 = E(U|T_1)$ and $\displaystyle U_2 = E(U|T_2)$. Show that $\displaystyle U_2 = E(U_1|T_2)$."