Is it possible to Raze the 2nd capital?

Solution 1:

Yes, you can raze a civlization's "second" capitol, i.e. the city that the capitol is moved to once you capture their original capitol. You can't raze a civilization's original capitol, because a domination victory means "owning all civlizations' original capitols."

To find out whether a city is an original capitol, go to the Victory Progress screen. (It's one of the options when you click on the scroll icon near the upper-right corner.) In the Domination section, you can mouseover each civilization's symbol, and the title-text will tell you what that civilization's original capitol is, and who the current owner is. (Alternatively, you can simply memorize the name of each civilization's capitol city; it's always the same, unless you've been renaming cities. So Germany's capitol is always Berlin.)

Solution 2:

No. The "no razing the capital" rule exists for game balance. It makes it so that a nation can't be completely locked out of a domination victory because their original capital was razed. Their new capital has no meaning for domination victory conditions, so it can be raised as usual.