Contemplating about a second PhD in statistics/machine learning
As other people already stated, multiple PhD's is almost certainly not a good idea. How to transition into machine learning depends on where you want to work:


academia, you can try to get a postdoc position in machine learning, in that way you can build your career by writing publications and get up to speed with machine learning during your postdoc research. This could be problematic as you might have to compete with people that have a PhD in machine learning. But this can be overcome during the interview stage. 
industry, you can simply apply to entry level machine learning positions (look for data science positions) and take it from there. Once you convince a company that you are a worthwhile addition, you can learn the ropes on the job.


In general, I would recommend already getting up-to-speed a bit in machine learning, try some online tutorials, etc. This can really help in securing a new position in either academia or industry.