Status of Final Doom

Around here is Final Doom generally considered to an individual game in its own right? Or is it considered to just be a pair of commercial stand-alone megawads?

Final Doom is an individual game in the sense that it was sold individually and will run as a stand-alone game, and it is also a pair of commercial stand-alone megawads. Both of those characteristics are rather evident, and are not mutually exclusive, so I'm not sure what other kinds of responses you were expecting here. Could you clarify your question, perhaps?

Umm well Final Doom is one game, it just so happens to be comprised of two megawads.

For me Final Doom is two separate games : Evilution and Plutonia (the only one that I have enjoyed in this pack).
So, between Doom2 and Final Doom, there are three separate games imo :P

I can't call TNT and Plutonia the same episode, just because they're very different. TNT focused on new graphics/music and innovative level designs (or something like that) whereas Plutonia was a challenge in both layout and difficulty of the monsters present. Both great WADs in their own ways, but certainly not two parts of the same episode.

Well if you look at it the way the above two stated it then I would say it's two separate games, they don't necessarily go right into one another... so yeah.

It's two separate games. They have two separate storylines. I guess it's kind of analagous to the episodes in Doom 1.

Evilution and Plutonia are two separate works as far as authorship goes*, although they always come together commercially (they constitute one item from the publisher's standpoint). In the strictest sense it's best to treat them separately, but it's not like with DOOM and DOOM II (and in relation, Final DOOM as a whole) that could and can be obtained separately.

* The Casali brothers were also part of Team TNT and involved in Evilution, but Plutonia is solely theirs.