Updated Two domains, same content, different theme

Greetings Everyone!

I am a huge fan of WordPress and how open everything is and most problems I can usually find an answer for. However, This problem has me a bit stumped as any answer’s I am finding either don’t fully apply or for older wordpress’s and no longer work.
(and I have tried searching)

Problem:
I have a friend who has site SiteA.com running on wordpress.
He now would like me to setup SiteB.com with the exact same content that SiteA has but with a different theme and no extra work.
So when he makes, edits, or deletes a post on SiteA.com then the same thing happen’s on SiteB.com (Sounds like using the same DB). preferably also using the same credentials (but that’s not a huge issue if it doesn’t, mainly just content mirroring)

I have a background on DataBases so getting my hands dirty on SQL code won’t be an issue there but as how I will get wordpress to do this has me a bit lost.
Also, I am aware of the potential SEO hit.