Your point that the two creation accounts are different stories written at different times is well taken, and many people underappreciate that fact.

However, it is also important to recognize that they were brought together and redacted by an editor (or committee of editors, perhaps even spread out over time) that arranged and tweaked the stories. At the very least, it is easy to glean from the text that they are intended to form a whole "collection," with connections between the various myths. The textual evidence suggests that the connections are stronger than that.

So while it is the more common error to underappreciate the differences between the accounts, it is a less common but equally erroneous mistake to underappreciate the "unity" (of a sort) that they have.