User blog comment:Poptropica411/Phineas plus Isabella equals Phineas and Ferb: Across the Second Dimension in Fabulous 2D?/@comment-1647442-20100809183307

Well yes, the Second Dimension is another dimension. A parallel one. The official summary:

Phineas and Ferb discover that their beloved pet platypus, Perry, is really a secret agent who battles the forces of evil daily. The boys team up with Perry on a grand adventure through a parallel universe where they must ultimately thwart an even more evil version of Dr. Doofenshmirtz.

So it's a parallel universe/dimension/whatever you wanna call it