Dual AC/20’s headcapping niche is /was targets with full head armor and 40% DR. Typically you’d pack some SRM2s to significantly up the odds of pilot damage if neither shot hit the head, and maybe a medium laser or two for buildings/ammo conservation, plus jump jets.
You take the shot from the side, so even if you don’t instakill the Target you still mess up one side. You also have a “normal” headcapper like a Stalker or GRH, plus an LRM boat. If the target doesn’t have entrenched, you can knock it over with LRMs pretty easily (assuming you’re running AC/20+++) If it does, you can instead follow up with a second called shot to the leg on the weakened side, scoring the knockdown + ST destruction for 2 more pilot injuries and typically removing most of the target’s firepower. Depending on the type of pilot you still need 1-2 more pilot hits, but there’s still a side torso to destroy and the mech will now immediately fall if you fill up the stability bar, and you usually haven’t hurt the CT too bad so you’re unlikely to kill the CT if you go for a headshot with SRM/MG.
The 2X AC/20 build still set its lancemates up for a kill if the 1/3 head instakill didn’t come up.
How that's better than just doing two consecutive salvos aimed at the head from the front, be it from the same mech or two different ones?. It's simpler, you can do it on the go, earlier, no LRMs or other particular requirements but two mechs with a good chance of blowing the head for around 60% of headcapping at 40% DR (or one headcapper and one long range finisher). The follow up, if required, can be anything really, from a long range direct damage mech, LRM boats, another headcapper, a trainer mech, anything which can do a lot of damage as a failsafe, it can be a non stab damage loadout.
In addition to be faster you can keep your forward mechs grouped together, so it's safer too. You don't need a lot of preparations, although this is better done after jumping from out of LoS, which is not a requirement but only a good practice in general, and not just for headcapping.
Then, those two mechs will have better armor, better cooling and better mobility. Because the two AC20s are very heavy, anything you add on top of them will have an impact, and remember they have significant recoil too, you may want to account for that.
Then, the hits that miss the CT, those have slightly lower chance to hit the CT than with regular salvos, the CT doesn't get preferential treatment when you miss the head (that would be clustering), and as Pherdnut says is much easier to tune down damage with a full ML/SRM setup, it's way more predictable, extremely unlikely to CT core with two salvos aimed at the head, even with three. But let's say I do it your way, how exactly is better doing it with AC20s than with non AC20 setups, the same tactic?. Because I don't see anything in your reasoning which makes any difference.
Just remember, 2 AC/20+++ is/was way easier to get together (Demolishers) than 6 ml++d and 4 srm6+++ or 7 ml++d and 6 sl++d, and without all the ++d weapons it’s a lot harder to get through the CT at 40% DR.
Not really. Against vehicles the most important thing is raw firepower and SRM+ML setups have plenty. If one of your AC20s go to the front and the other to a side you may not kill him (each side has almost the same chance as the front without PS), if one of them misses you won't kill him. In fact you won't do the demo if it's in cover and both shots land at the front plate. You don't have damage enough for that unless you add more firepower on top of the 2xAC20, weapons that will compromise your general survivability, cooling and mobility, whereas with a LRM boat you'll kill him every time.
And of course without ++ weapons the AC20 gets a lot better in comparison, but I already agreed AC20 is very good early on. But at that stage headcapping is not really a thing. It can happen but there are no "real" headcappers, mechs capable of semi consistently headcapping without full team effort. That's more of a late game thing.