Sonics 1 and 2 were not among the Megadrive games re-released for MegaCD in collections or enhanced versions. Though Sonic 2 was pitched as cross-platform, "Sonic CD" became its own unique entity. When hobbyists speculated that the resources, including 2p Tails, were too much for the MegaCD to handle, I took on the challenge of learning about the hardware and breaking down Sonics 1 and 2 for this purpose. This MegaCD Proof-of-Concept demonstrates the first zone of each, including "Knuckles in [...]" enhancements, Tails with player-controlled flight in both games, and a custom level with CD audio, currently the only existing custom level whose data meets Sonic 2's split-screen requirements.
This was originally released in 2006. As noted in the included documentation, certain hardware issues that were not documented or known at the time, or to this day represented in emulators, rendered sound inoperational in Sonic 2, and I didn't have the have the time that it would take to implement a proper workaround.
This project, itself, exists only as a proof, and was not meant to be completed. To find out what became of the concept, check out Sonic Megamix.
For a few technical details regarding this project, see this blog entry.