How to answer a supervisor's email that includes and presents another professor to you?
It depends on what that new contact is for.

If it is for something completely unrelated to anything that you are working with professor A, then I'd suggest replying to both with a thank you message and then separately contacting Prof B.

If, instead, the work is, even sidely related to whatever you are doing with Prof A, add everyone in your emails. Transparent communication is key in any healthy relationship. If you are worried about spamming Prof A with relatively non-related content you can always contact Prof A separately and directly ask him if he/she wants to keep being CC'd in the email chain, there is no problem with that.

These Profs, they are just people, treat them like people!. Don't overthink it, as StrongBad says.