Which specialisation should I opt as minor in the 2nd MBA, marketing or operations?

Since you have opted for HR as major, marketing is the best suitable as minor. Operations shall be totally different and would have no relevance with HRM.

Go for Marketing as major and choose Finance or International Business Management as minor. This combination will fetch you good placement opportunities in future, plus will consolidate your position in corporate world as a whole. Good luck.

It should actually depend on your strengths. If you can talk well, travel, are creative and can persuade and negotiate, go for marketing.