I'll take this challenge.

Okay, so on the surface level it appears to be similar, but once you actually go beyond that they're nothing alike.

First- Twins: The Test sisters are basically the same character, to their likes, dislikes, and personality. they might as well be the same person. But then the "writers" couldn't have then fight over Gil when they some how have a chance.

The Older Pines twins, are very different from each, when they were younger they were close. But developed into different people as they grew up. To remove one of the twins would completely change the dynamic of the show.

Second- The Agents: Again similar to the twins could have been one character. But they do act differently enough to warrant separation. They're idiots with lots of government clearance, when they really shouldn't, and are obsessed with vacationing in Fiji. Also they're named opposite of their race…..because that's oh so clever…..

Agent Powers and Trigger. Serious, determined, and professional. They have no real motivation but to follow through their directives and solve the case. Unlike Black and White in Test, Power and Trigger aren't frequent characters only appearing to set up the fact the US government is concerned about Gravity Falls.