Yet—and it’s obvious where I’m going with this—“Spock murdered EvilKirk” is not a meme. ...[W]hy does he get a pass while Janeway is condemned?

I'm gonna cheat here a little bit. There are lots of things about the Tuvix debate that really are not about Tuvix; we've seen Tuvix elicit underlying opinions about everything from abortion to the trolley problem, and we've often seen thinly-veiled misogyny lurking beneath the surface as certain folks appear interested in finding any flaws in Captain Janeway they can.

So, yes, somewhat trivially, I think that Spock does not get condemned in part because he enjoys the luxury of not being targeted by misogynists. I think it's a small part, but surely a part.

I think the reason there aren't memes is because, well... "The Enemy Within" is an old episode, it never attracted attention through debate, and also it's a gross episode where EvilKirk straight up attempts to rape Yeoman Rand, and then Spock makes a joke (! Spock! A joke!) about it at the end of the episode. I'm sure it's an episode numerous folks have tried to forget. (I know I have.)

So, in terms of using the episode as a barometer to evaluate our own reactions to the two situations, I think there are overriding contextual factors that drown out any insight we might gain.

As an aside, I also think there is a pretty straightforward argument that EvilKirk (and GoodKirk, for that matter) was not mentally competent. He was, by definition, the remnants of an individual who had had a significant piece of their person torn away from them traumatically.

I think there's actually an odd but useful comparison to dementia here: dementia does cause some individuals to behave "out-of-character", immorally, or just meanly. If there were a "transporter reintegration" equivalent to treating dementia, and the patient said, "No, I want to stay like I am"... then I think probably the patient's wishes would be ignored.

Part of the reason "Tuvix" is hard is because, at least superficially, Tuvix appears mentally competent, so it's much harder to justify ignoring his wishes.

(As a second aside, one thing that always surprises me about the Tuvix Discourse™ is how little attention is paid to Tuvix being... well, somewhere between an asshole and a creep. He is incredibly manipulative toward Kes, preying on both her feelings for Neelix as well as her mentoring relationship with Tuvok. Neelix of course always was a bit possessive and jealous, but he at least was written "with a good heart"; I felt like Tuvix took those same qualities, but added a Tuvok-esque cold calculation to it. In any case, to me there's a mildly interesting parallel between these two episodes where there's this tone-deafness to the way the writers treat the behavior of the "transporter accident individual.")

why does he get a pass while Janeway is condemned?

Firstly, the episodes are doing completely different things, and have completely different presentations. "The Enemy Within" uses the transporter malfunction to examin the duality of man, and doesn't address the ethics of the situation in any way. That's going to inform the viewers' reactions, just as bringing Janeway's decisions regarding Tuvix to the forefront of that episode informs the viewers' reactions of that.

Secondly, Tuvix himself would have agreed with Spock - at least, at first. He was initially an active participant in trying to find a way to undo the situation. Over time, though, he changed...and so did Janeway and the Voyager crew. Tuvix is given a name. He's given a job. Janeway calls him an officer and an advisor.

In short, Janeway granted Tuvix personhood...and then unilaterally stripped it away.

Isn’t the main difference that neither could survive if they remained separated? They would both eventually die unless they were reintegrated.

Whereas in Tuvix’s case, the merged being was in no danger of death and could have lived a long healthy life.

I would say Neelix is a big part. Starfleet officers have already consented to the possibility that they might be ordered to their death for the good of their ship or the Federation. While their transformed versions may not have been exactly the same beings as the originals, there is still some ethical cover in that some version of them in the past had accepted these risks. Neelix, however, was not a Starfleet officer and had made it very clear that he did not consent to having his life sacrificed for the greater good of Voyager.

Just how much this ethical baggage from either side carried over to a new being is unclear; it certainly would have been less unclear if it had been two officers merged together and not a civilian passenger.

I always got the impression that the Kirks were both destined to wither away and die if kept separate, but it's been a while so I might be wrong there.

There's also the issue that EvilKirk was a danger to everyone else on the ship. You could make the argument that if he survived, he could have learned to control his behaviour, but I'm not sure that's the case for a personality entirely made up of those negative traits.

It's definitely a moral grey area, whereas with the Tuvix situation it's more black and white. There was no danger to anyone else, he was well-liked and a credit to the ship, there was basically no reason for Janeway to split him back apart other than "I want to".

Well, (and this isn't exactly an answer to your question), one thing that's interesting to consider is... if Spock killed Evil Kirk, he also killed Good Kirk. In other words he killed two people to save one (if you want to look at each of them as individuals in a sense equal to the original). Janeway "killed" one person to save two.

Janeway made the right choice with Tuvix. It might have been murder but the alternative was double murder