[S3K] 2nd player CPU trigger

I'm not quite sure what you mean, but if you want to know how the game chooses which characters to select, it's through the flag, in RAM, at $FFFF08. If this is 0, the game initializes Sonic and Tails objects, if it's 1, it only initializes Sonic, if it's 2, Tails, and if it's 3, Knuckles. The player initialization routine, at $6A76, handles player object initialization on the basis of this flag.

 

Play sonic 3 delta, I think you can have tails follow knuckles on it.

 

There's no separate flag to tell Tails to appear, Tails only loads as a second player when $FFFF08 is 0. However, what you can do is replace the rts in Knux alone's loading routine with a branch to the Tails Sidekick loading routine. Unfortunately, the branch distance (-$88) is too big to fit in a short branch, so you'll have to overwrite the following instruction, which will mess up slot machine bonus stage loading, but anyway, to do it, just replace the 4E75 at 6B74 with 6000 FF78.

EDIT: To clarify, what that does is, instead of returning from the subroutine, it branches back to the place where Tails' initialization is handled (inside the Sonic & Tails initialization part). It's much simpler to do stuff like this in ASM, although I think there are a lot of alignment directives which have to be placed inside Stealth's disasm for it to be able to work with location shifts.

 

The first object in RAM (player 1 during levels) is stored at $FFB000, the next one at $FFB04A. The first four bytes of each object in RAM are a long pointer to the object's code, so if you want the game to load Knuckles instead of Sonic you'd have to change the longword at $FFB000 from $00010A94 to $00016444. This will cause Knuckles to be blue and the game will still think you're Sonic, but Tails will be following you.
Alternatively, you could change the second object in RAM to be Tails during a Knuckles game. To do this you need to write $0001365C to $FFB04A. Tails won't be there right away, but he'll fly in from above soon enough.

Be warned, though! If you change these anywhere outside a level the game will crash.

Edit: wait! Are you using ASM or hacking RAM values with AR codes?

 

So will you charge people to watch you do these "other things" to it?

 

Did you try this already:

Tails's position only gets set during player mode 0 (Sonic and Tails), so that's why he's not there right away.
It will be very hard to get this working 100% with RAM hacking alone.
Oh, and, go you, if you're making an ASM hack of s3k. It's about time.