I signed up for the Gold challenge, then about two weeks later signed up for Platinum (did not mention my previous Gold signup in that call).

Both challenges stuck. I achieved GLD on the outbound trans-Atlantic segment of a DCA-LGW RT, and PLT on the intra-US leg of my return. I got challenge completion bonuses for both of those legs that made my mileage equivalent to taking those flights as the new status, and I got the standard GLD bonus for the trans-Atlantic return flight taken as a GLD.

So I achieved AA PLT in 4 segments. That's a heck of a way to start out the New Year.