installation details

I want to set up a ring pro doorbell with my current home system. I have one push button that rings two bells uses a 16V 15VA transformer. Two questions: 1. do I need to upgrade my transformer? to 16V30VA or 24V40VA? 2. How do I wire the power pro kit? Specifically to either of the bells? and from your website it looks like the bells must be in series, if they are in parallel do I need two power pro kits? Having the two bells and the ring pro, will that suggest using a bigger transformer?