I think this will be on Relapse 2. Why? The original says "see you in Hell for the sequel," implying that there'd almost certainly be a sequel. Unfortunately, Royce & Em had a falling out but then patched things up & then Royce contributed to Eminem's record.
On Wikipedia it states: "Royce has stated that the sequel to "Bad Meets Evil" (alluded to in the first part of the song with the line "See you in hell for the sequel") may be recorded and released on Eminem's upcoming 2009 seventh studio album, Relapse 2."
This is great news. Royce brings out the best in Eminem, lyrically. Because Royce is so amazing, Eminem feels like he has to top him. That's how we get the poetry of "Renegade" (widely regarded as Eminem's dopest verse), "Nuttin' To Do," "Scary Movie," "Rock City."
Imagine a sequel Bad Meets Evil II, ten years after the original.