public class nnn : MonoBehaviour
{
    [Serializable]
    public class Shit<T, K>
    {
        public T val1;
        public T val2;
    }

    void Start()
    {
        var shit = new Shit<string, string>()
        {
            val1 = "kkk",
            val2 = "kokkj"
        };
        var serialized = JsonUtility.ToJson(shit);
        Debug.LogError(serialized);
    }
}