Sonic & Knuckles Lock-On ROM Maker This tool simulates a Sega Genesis game being locked onto Sonic & Knuckles by combining their ROMs similar to how it would be done on a real Sonic & Knuckles cartridge. To get Knuckles in Sonic 2, you will also need the Knuckles in Sonic 2 UPMEM ROM, which will only be applied if a Sonic 2 serial number is found in the locked-on cartridge ROM (unless you set the option to force it to be applied). Do note that this isn't a 100% simulation of how the lock-on actually works. That involves a mapper which can enable and disable SRAM and the Knuckles in Sonic 2 UPMEM ROM on the fly, which can't be totally simulated by just combining ROMs. This is merely designed to be a quick and easy way to mimic the lock-on by creating a combined ROM that you can run in any Sega Genesis emulator. Luckily, with the Knuckles in Sonic 2 UPMEM, that only really gets run when a cartridge with a Sonic 2 serial number is locked on, and doesn't get turned off. With SRAM, I've added an option to inherit the SRAM settings from the locked-on ROM's header, so that it can be detected properly (though, Sonic 3 & Knuckles is usually checked by emulators, so you shouldn't need to inherit Sonic 3's SRAM settings). That setting is only any useful if you use an exploit to start executing the locked-on cartridge's code and manage SRAM from there. Feel free to send in bug reports or feature requests here.
