Let \( e_1 \) and \( e_2 \) be the eccentricities of the ellipse \( \dfrac{x^{2}}{b^{2}} + \dfrac{y^{2}}{25} = 1 \) and the hyperbola \( \dfrac{x^{2}}{16} - \dfrac{y^{2}}{b^{2}} = 1 \), respectively. If \( b < 5 \) and \( e_1 e_2 = 1 \), then the eccentricity of the ellipse having its axes along the coordinate axes and passing through all four foci (two of the ellipse and two of the hyperbola) is:
