  We evaluated the bias and variability on the fitted age and convective core
overshooting parameter for evolved binary stars accounting for observational
and internal uncertainties. We considered a binary system composed of a 2.50
$M_{\sun}$ primary star coupled with a 2.38 $M_{\sun}$ secondary in three
evolutionary stages (primary at the end of the central helium burning; at the
bottom of the RGB; and in the helium core burning). The simulations have been
carried out for two values of accuracy on the mass determination (1% and 0.1%).
We found that the fitted age and overshooting efficiency are always biased
towards low values. The underestimation is relevant for a primary in the
central helium burning stage, reaching -8.5% in age and -0.04 (-25% relative
error) in the overshooting parameter $\beta$. In the other scenarios, an
undervaluation of the age by about 4% occurs. A large variability in the fitted
values between simulations was found: for an individual system calibration, the
value of $\beta$ can vary from 0.0 to 0.26. For an error of 0.1% on the masses
the global variability is suppressed by a factor of two. We accounted for a
systematic offset in the effective temperature of the stars by $\pm 150$ K. For
a mass error of 1% $\beta$ is largely biased towards the edges of the explored
range, while for the lower mass uncertainty it is basically unconstrained from
0.0 to 0.2. We evaluated the possibility of individually recovering the $\beta$
value for both stars. We found that this is impossible for a primary near to
central hydrogen exhaustion, while in the other cases the fitted $\beta$ are
consistent, but always biased. Finally, the possibility to distinguish between
models computed with mild overshooting from models with no overshooting
resulted in a reassuring power of 80%. However, the scenario with a primary in
the central helium burning showed a power lower than 5%.
