Let each of the two ellipses \( E_1:\ \frac{x^2}{a^2} + \frac{y^2}{b^2} = 1 \ (a > b) \) and \( E_2:\ \frac{x^2}{A^2} + \frac{y^2}{B^2} = 1 \ (A < B) \) have eccentricity \( \frac{4}{5} \). Let $\ell_1,\ell_2$ be lengths of latus rectum of $E_1,E_2$ respectively such that $2\ell_1=\ell_2$. If the distance between the foci of $E_1$ is $8$, then the distance between foci of $E_2$ is: