Rationalization is probably the one that was the most enlightening. I tend to sometimes (knowingly) think in that way, but didn't necessarily view it as a negative thing (and didn't know the term) before reading the tag description here. Confirmation Bias seems like it's related to (or could be a part of) rationalization as well. Another one that stood out was Akrasia.

These are the ones that stood out to me initially. Looking now at the Concepts page again, I feel like I should at some point check out all of the terms at least by hovering over them, because there are a lot of ones that look interesting or may have a deeper meaning than they might seem to at first sight.

It appears that there is a typo in the first sentence:

This is a new FAQ written LessWrong 2.0.

I would expect it to read something like "This is a new FAQ written for LessWrong 2.0." instead.

I agree that deniability from a technical perspective (such as absence of a direct link from the account on one platform to the account on the other platform and vice versa) doesn't necessarily make it much harder to identify that it's the same person. On the other hand, even if the username, profile picture, etc. are exactly the same, one needs to be careful about associating the accounts too quickly, especially in the case of some common/simple username—it could just be a coincidence.

It seems that things like behavior and writing style are more important to keep similar or distinct—it would take a little while longer to pick them up for an outside observer, but they seem to be much more reliable signals for linking accounts; even if the user in question denies that both accounts belong to them, I think it's more likely that the observer just wouldn't believe them if they know the user's writing style.

Who are you trying to confuse into keeping your identities separate?

This is a good question, I don't really have a specific threat model in mind. Obviously, I want "the bad guys" to know as little about me as possible, but I can't seem to easily define who they are and how I would identify someone as belonging to them before I had a chance to talk with that someone or at least observe their behavior for a while. Regarding the quiz idea in my question, them having similar interests (or knowing mine) doesn't guarantee that they aren't part of "the bad guys" either.