When mousing over a civilization that was forced into capitulation, you'll see information about what % of population/land the vassal has compared to its master. However, you don't see any such information for colonies, and the rules seem different. Reading over the civilopedia, it seems as though the only way they'll split, is if relations get really bad between the two.
In my current game, Gilgamesh colonized part of a second continent, and apparently moved his capital. When he created the Babylonians as a colony, they actually took over his starting position, with Uruk as the capital city. Now, Hammurabi has a slightly higher score than Gilgamesh, but is still considered a colony.
One reason I'm asking is for my current game; Joao and I used to be buddies, but he's been a pain lately. At one point he declared war on me, but there was a forced peace from the Apostolic Palace. I wouldn't mind stomping him if he tries it again, but I don't want to be at war with his American colony (with whom I've got really good relations). I don't wanna cheat, so it may come down to beating up on Joao, and then trying to make nice with Lincoln afterwards.