Ph.D. in Geophysics while doing an online MS in CS
In general, the CS and machine learning community is more interested in what you build than in what diploma's you have. There won't be many companies where a masters degree is going to significantly compliment a phd with relevant practical experience. A masters degree could give you a more solid theoretical foundation and the possibility to work on some other (toy) project giving you more broad experience. However you can also get that using coursera and personal projects.  

The real trade-off is: a more structured program versus a more self-made learning path. The degree program also 'forces' you to do the work, which can be a good thing or a bad thing.