The ParseStep would need to access the active text editor control. You would also need to attach the CodeCompletionProvider and ToolTipProvider to all the text editors. You would also need to use a different filename for each text editor.
Alternatively you could look at the snippet compiler sample that was recently added to SharpDevelop 3.0. This has support for multiple text editors and debugging. If you don't want support for debugging then it is probably easier to modify the CSharpCodeCompletion sample to support multiple text editors.