SecondaryRender.cs

using UnityEngine;

public class SecondaryRender : MonoBehaviour {
    void OnPreRender() {
        foreach(Renderer renderer in FindObjectsOfType<Renderer>())
            if(renderer.gameObject.layer != LayerMask.NameToLayer("Secondary"))
                renderer.shadowCastingMode = UnityEngine.Rendering.ShadowCastingMode.ShadowsOnly;
    }

    void OnPostRender() {
        foreach(Renderer renderer in FindObjectsOfType<Renderer>())
            if(renderer.gameObject.layer != LayerMask.NameToLayer("Secondary"))
                renderer.shadowCastingMode = UnityEngine.Rendering.ShadowCastingMode.On;
    }
}