I never knew there was a "Tails can fly forever" one

I'm definitely trying that one out

wait,what game are these codes 4?

Sonic 3 & Knuckles. Says so right there in the thread title. Also I'm fairly sure S3&K is the only game to feature Super Knuckles or Super Tails.

Hate to be pedantic but S&K and S2&K also feature Super Knuckles. 😛

Anyway, on topic, I just found some amazing S3&K codes on gscentral.org. Some of them I haven't gotten round to testing yet, but all the codes I've tried so far have worked.

ABJT-CACA - 0 emeralds needed for Super Tails
ABJT-CACJ - 0 rings needed for Super Tails
NVJT-CACJ - 100 rings needed for Super Tails
W3JT-CACJ - 150 rings needed for Super Tails
3BJT-CACJ - 200 rings needed for Super Tails
9KJT-CACJ - 250 rings needed for Super Tails
ABZT-AA5Y - Knuckles & Tails (1)
PKZT-B950 - Knuckles & Tails (2)

Yes. SERIOUSLY. Enable both the codes, start a game as Knuckles, and Tails joins you for a 2-player game. I've only tested this in Angel Island Act 1 so far - and it might work in S&K too. Sadly, it doesn't look like Tails can carry Knuckles while he's flying.

Untested codes (some might be unfunctional in S3&K but functional in vanilla S&K):

FFB038:00 - Sonic level layout
FFB038:01 - Tails level layout
FFB038:02 - Knuckles level layout
FFFF0B:01 - Play as Sonic
FFFF0B:02 - Play as Tails
FFFF0B:03 - Play as Knuckles
FFF761:6419 - Super speed (Sonic and Knuckles)
FFFEC1:6419 - Super speed (Tails)
AAPT-CAB8 - Super/Hyper doesn't use rings

Presumably the 'Play as....' codes allow you to switch characters mid-game, possibly using the original character's level layout (and possibly once enabled the code won't take effect until you reach the next zone). That could allow you to fight Knuckles... as Knuckles... in Hidden Palace.
That's an alternate Super/Hyper code to the one that I know works, but if I remember rightly the old code doesn't work for Tails, whereas the description on the website for the new code states specifically that it's for ALL characters.

A bonus unconfirmed code for Sonic 2 & Knuckles...

ADFW-8AF6 - Regular music for Super Knuckles

That should make the normal level music play when you turn into Super Knux, rather than the Super music (which gets annoying eventually...).

Well if you glitch your way to Sonic's fight in Marble Garden as Knuckles, Tails will carry you:

Slight language warning

Well if you glitch your way to Sonic's fight in Marble Garden as Knuckles, Tails will carry you.

True, but for some reason he won't during regular play using the Knuckles w/ Tails code. I haven't tried using the code in Marble Garden act 2 yet...

Note on the Knuckles and Tails code: If you play with this code enabled during the transition between acts in level 2, Tails won't follow you. Also, the second checkpoint sends you back to the main screen regardless.

Note on the Knuckles and Tails code: If you play with this code enabled during the transition between acts in level 2, Tails won't follow you. Also, the second checkpoint sends you back to the main screen regardless.

I've just done a more thorough testing, and Tails does stop following Knuckles in Hydrocity Act 2 (he doesn't get pulled down by the big suction thing after the first boss), but I've never been booted back to the main menu for using the code; at the end of Hydrocity I just went straight through to Marble Garden (where Tails continued following me as per usual).

Anyway, here's what I did find:
Marble Garden works fine, if you cheat for Knuckles to use Sonic's route through Act 2, Tails airlifts Knux for the boss as usual. However, I haven't found any other occasion where Tails' airlift works for Knuckles.
There are glitches, for example when I took Knuckles & Tails through Sonic's route of Sky Sanctuary, Tails was nowhere to be found until right at the end of the level, and even then he wouldn't move. However, he returned for the Death Egg behaving just as normal. And when I tried Knuckles' version of Sky Sanctuary, Tails appears just fine, but he freezes after Metallix' first defeat (however, I think the exact same effect happens when you take Sonic & Tails into Knuckles' Sky Sanctuary version).

Here's what I've found after testing the unconfirmed codes from my last post:
-The 'Play as...' codes don't work properly, all they seem to do is turn the player character into a blue Knuckles, regardless of which code you use or which character you select. However, Knuckles keeps all his special moves, and it IS possible to fight Knuckles as Knuckles using this code.
-The '...level layout' codes don't work at all, as far as I can tell.
-'Super speed' works just fine - it VASTLY increases player run/jump (and gliding I think) speed. It doesn't affect wall-climbing or jumping height. Haven't tried it with Tails' flying ability yet...
-The new 'Super/Hyper doesn't use rings' code works perfectly, but it turned out that I remembered wrongly, there wasn't any problem with the old code I'd been using (NTPT-CA50).
-The Sonic 2 & Knuckles 'regular music for Super Knux' code works perfectly, though the music for the level does restart when you transform.

And of course I'd already tested all the Super Tails codes, which all work fine.

NOTE: One other noteworthy glitch I found with the K&T code. After defeating the second boss with Knuckles & Tails in Sonic's version of Marble Garden (though this will probably happen if you face Knuckles' regular boss too) - Carnival Night Zone started off weirdly, if I remember rightly Knuckles and Tails spawned in Sonic's normal starting point, even though the screen was focussed on Knuckles' starting point. I did sort this out easily enough, but I can't remember if it was by moving around and jumping normally or by pressing 'B' in debug mode.

I should have been more specific. When you activate the second checkpoint (the one that you encounter after the section that if you don't move fast enough, the walls will squish you into a pulp) has this little glitch. I wonder also if using an emulator has anything to do with these codes.