According to IMDB, Batman will have some assistance in upcoming Batman: Arkham Asylum sequel - he'll be joined by Robin.
The listing on IMDB indicates Vincent Martella (Batman: Under the Red Hood animated feature), will be voicing The Boy Wonder. The introduction of Robin could indicate the inclusion of some form of cooperative play in the sequel.
IMDB also lists Bruce Greenwood as the voice of Two-Face.
Warner Bros. told Kotaku: "To our knowledge, what you asked about is inaccurate information. We are not yet discussing details for the sequel."
That doesn't sound like an outright no, more that it's not ready to talk about the game at this time.