要保存嵌套RecyclerView的滚动位置，可以使用以下步骤：

1. 在父RecyclerView的LayoutManager中保存子RecyclerView的滚动位置。
2. 在父RecyclerView的onSaveInstanceState方法中保存子RecyclerView的滚动位置。
3. 在父RecyclerView的onRestoreInstanceState方法中恢复子RecyclerView的滚动位置。

以下是一个示例代码：

public class ParentRecyclerView extends RecyclerView {
    private LinearLayoutManager mLayoutManager;
    private SparseArray mSavedStateArray = new SparseArray<>();

    public ParentRecyclerView(Context context) {
        super(context);
        init();
    }

    public ParentRecyclerView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public ParentRecyclerView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    private void init() {
        mLayoutManager = new LinearLayoutManager(getContext());
        setLayoutManager(mLayoutManager);
    }

    @Override
    protected Parcelable onSaveInstanceState() {
        Bundle state = new Bundle();
        state.putParcelable("superState", super.onSaveInstanceState());

        // 保存子RecyclerView的滚动位置
        for (int i = 0; i < getChildCount(); i++) {
            View childView = getChildAt(i);
            if (childView instanceof ChildRecyclerView) {
                int position = getChildAdapterPosition(childView);
                Parcelable savedState = mLayoutManager.onSaveInstanceState();
                mSavedStateArray.put(position, savedState);
            }
        }

        state.putSparseParcelableArray("childStates", mSavedStateArray);
        return state;
    }

    @Override
    protected void onRestoreInstanceState(Parcelable state) {
        if (state instanceof Bundle) {
            Bundle savedState = (Bundle) state;
            super.onRestoreInstanceState(savedState.getParcelable("superState"));

            // 恢复子RecyclerView的滚动位置
            mSavedStateArray = savedState.getSparseParcelableArray("childStates");
            for (int i = 0; i < mSavedStateArray.size(); i++) {
                int position = mSavedStateArray.keyAt(i);
                Parcelable childState = mSavedStateArray.get(position);
                View childView = mLayoutManager.findViewByPosition(position);
                if (childView instanceof ChildRecyclerView) {
                    ((ChildRecyclerView) childView.getLayoutManager()).onRestoreInstanceState(childState);
                }
            }
        } else {
            super.onRestoreInstanceState(state);
        }
    }
}

public class ChildRecyclerView extends RecyclerView {
    private LinearLayoutManager mLayoutManager;

    public ChildRecyclerView(Context context) {
        super(context);
        init();
    }

    public ChildRecyclerView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public ChildRecyclerView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    private void init() {
        mLayoutManager = new LinearLayoutManager(getContext());
        setLayoutManager(mLayoutManager);
    }
}

在示例中，ParentRecyclerView是父RecyclerView，ChildRecyclerView是子RecyclerView。在ParentRecyclerView的onSaveInstanceState方法中，保存了子RecyclerView的滚动位置，并在onRestoreInstanceState方法中恢复了子RecyclerView的滚动位置。