This varies wildly, not just from publisher to publisher, but from one individual creative team to another. But for the most part, yes, collaborative efforts like this have been common in the comics industry for decades (at least as early as the 60s, it's hard to pin down given the lack of credit given to many of the people involved in the comics process in the 30s and 40s). In fact, it's common for there to be four roles involved, a writer who writes out what happens in the book and the dialogue, a penciller who does the initial artwork, an inker who goes over the initial pencil sketches and does the ink line-work, and a colorist who colors the linework. Sometimes one of those folks will also add the word balloons and lettering, sometimes this is done by a fifth person, a letterer. And some series will have multiple people filling each role. On the other hand, sometimes these roles will be combined, with as little as one or two people handling an entire book. And of course, as you noted, it was natural for one person to develop more than one of these skills, so he could be the penciler for one comic, inker for another, and colorist for a third (hopefully not all at once!). The nature of a monthly comic series often means that it's more efficient to have a sort of assembly line setup like this, where a page moves from person to person until it's complete, while the pencilers, inkers, and so on can continue working on the next page, and the page after that.

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