I know that currently the document area can be split vertically or horizontally (by right clicking on the file's tab and going to split), but I think it would be awesome if two different documents could be viewed at once.
To choose which document is in which window I would suggest just duplicating the tabs. Thus it should be possible to have the same document open twice, which is currently the only option when splitting your document.
Also would be cool (but maybe beyond Code::Block's goals) if the view of a document could be toggled between: plain text, syntax highlighted code, and hex.