Yeah, that is the ideal target market, and they will buy them both. But the people not yet "sold" on TBS games are not likely to get both. They'll get one. And CivV is currently the louder, as it's an established IP, and CivII caused an entire generation of students' GPAs at MIT to drop by half a point.
But this time, you have an edge. Galactic Civilizations 2 came out months after Civ IV, but Elemental is slanted to be released months before CivV.