Answer
$50 \mu m$.
Work Step by Step
See the ray diagram. Rays from each of the two objects, separated by L, pass through the cornea to the retina. The distance between the final points on the retina is $4 \mu m$, since the final points are at the center of cones that are $2 \mu m$ in diameter, and the 2 cones are separated by a cone that is not excited.
Note that we have similar triangles. Use this to solve for L.
$$\frac{L}{25cm}=\frac{4 \mu m}{2.0cm}$$
$$L=50 \mu m$$