Amex's "rule," apparently, is more-or-less that you cannot get a bonus for the same card twice.
However, in practice, it can sometimes be done with an approximately two or more year wait between cards. YMMV.
If you have to have things written out for you in the card company's rules you are going to have to pass up a lot of miles earning/churning opportunities.
Remember, never NEVER call the card company to confirm things you learn on FT. @:-)