NO CARRIER has written the following instructions of which I have gramatically corrected =). The kid can't spell or write, but he sure is enthusiastic! =)

Adding the Sonic and Knuckles Lock-On Technolgy with Sonic 2 and 3.
By No CARRIER
Logo-Free for your reading enjoyment.
Revision 1.02
(Sorry for sloppy layout!)
Lastest revision should take care of that!

How to play Sonic 2 & 3 w/ Sonic & Knuckles:

With a ROM copy of each Sonic the Hedgehog game at hand, and a stable Genesis
emulator ready to run, I guess it is time for my to put that bulky Genesis
and all those cartrages in the attic once and for all. Looking over the cartrages, 
I realized something, How am I going to be able to play Sonic 2 and 3 with 
the Sonic and Knuckles 'Lock-on Technology'? Hoping someone else would
provide the answer, I waited... and waited.. and decided, Hay, maybe I can
figure this thing out myself! -- and I did!

What to do:

It is quite simple. First get a ROM copy of the three games. Then, at a DOS 
prompt, type:

COPY /B S&K.BIN + SONIC3.BIN SONIC4.BIN 
           |            |            |
           |            |            - Output name (Can be anything you wish.)
           |            |
           |            - Name of Sonic 2 or 3 Rom
           |
            - Sonic and Knuckles Rom name

You MUST do this at a DOS prompt. The /B does a binary copy. As far as I 
know, there is NO way of doing this from FIle Manager. (Thats why I choice 
the good old DOS system to run Console emulators over Windows.)

Why it works: My theory
Before I begin explaining why I think it works, we must make 2 assumptions.

1.) The program code and the modified graphics for Sonic 2 and 3 are located
on the S&K cartrage. (I thing this is true because I don't see how the 
modifed graphics and all the changes in the program can be implimented on a 
Ready Only Memory medium.

2.) When you add a cartrage to the S&K cartridge unit, the Genesis treats it
like one cartridge with a heck of alot of memory. In the sense of the ROM 
images, it is just one large file. (Note, by combining them, the total size
of the two cartridges is smaller than the largest ROM size possible that the 
Genesis can control.) Remember, it is doing this without the aid of 
additional handware, just software.

At power-on of the Genesis, and after all the standard stand procedures are
run, (Like master reset, ect.), the S&K cartrage looks to see if there is
anything in the add-on port by trying to read the contents of the first bank
or so.  (It assumes that the smallest ROM size is under 1 Megabit or lower.
So, by accessing a bank in this area assures that it is going to read something.)

If something exists, it will get back some kind of feedback.  If it doesn't,
it will get back all 1's or 0's because there are no banks and it considers
them blank. (The output would more likly be  floating high 's.   I.e. all
1's)  Since there is cartridge in the add-on port, it runs the Sonic and
Knuckles game.  If it determines that add-on port is occupied, it will read
the header of the cartridge. (Think of the head something like a file name:
It's a means of identifing the cartridge if no label was present.)

If it is not Sonic the Hedgehog 2 or Sonic the Hedgehog 3, then it spits
out the message Ya right.  If it IS Sonic the Hedgehog 3 then it runs
Sonic 3 program on the S&K cartridge.  It uses the Sonic 3 cartridge for all
its graphics and sounds, except for any new ones.

At this point, if i iss not Sonic the Hedgehog 3, then it can only be Sonic
the Hedgehog 2.  So it runs the Sonic 2 program on the S&K cartrage. It uses
the Sonic 2 cartridge for all its graphics and sounds, except for any new
ones.

It doesn't work: What do I do?

From my tinkering around, Sonic 3 works fine with the S&K modifications, but
you cannot save your game. Sonic 2 does not work at the moment.  More likely,
it is the emulator.  Also, check to see if the header says Sonic the Hedgehog
2 or ...3. Those console game distributers like to change the Header with
things like Sonic 2 for Console Zone or Sonic 2 supplied by ^BaTMaN^.
Under my theory, this WILL cause problems. Find a program that lets you
change headers of carts.

As for getting into the secret bonus level by using another cart other than 
Sonic 2 or 3, for the S&K cart:

No luck either.