Ernie as Phineas and Bert as Ferb

According to AverageJoeArtwork, Ernie suits Phineas and Bert suits Ferb and vice versa because of the following reasons:

Ernie as Phineas

 * Both characters wear horizontal stripes and are talkative and fun-loving
 * They sing more than Bert and Ferb do.

Bert as Ferb

 * They both have "er" in the middle.
 * Both are on the opposite quiet side (Ferb doesn't talk that much).
 * They both have similar body structures and features (taller eyes, hair tufts, and manly outfits).