  We determine the exact value of the optimal symmetric rate point in the Dueck
zero-error capacity region of the binary adder channel with complete feedback.
Our motivation is a problem in quantitative group testing. Given a set of $n$
elements two of which are defective, the quantitative group testing problem
asks for the identification of these two defectives through a series of tests.
Each test gives the number of defectives contained in the tested subset, and
the outcomes of previous tests are assumed known at the time of designing the
current test. We establish that the minimum number of tests is asymptotic to
$(\log_2 n) / r$, where the constant $r \approx 0.78974$ lies strictly between
the lower bound $5/7 \approx 0.71428$ due to Gargano et al. and the
information-theoretic upper bound $(\log_2 3) / 2 \approx 0.79248$.
