Directional Light Bias problem

I have a problem in setting the best for the lights / shadows. I have a movable directional light that simulates the sun and to avoid strange effects on the ground requires a high bias (0.6 - 0.7), the rest of the objects, however, are much smaller and with such bias, the shadows are quite differed. I tried to add a new directional light with different culling mask and a lower bias (0.15), but this prevents me from seeing the shadows (of the mesh belonging to different culling mask).

Does anyone have a solution? perhaps also to avoid having the ripples on the ground with the lowest bias?