Well what would be the point ? They serve very different audiences. We would get hardly any domestic news at all, only news that would be relevant to the rest of the world !
BBC News 24 is aimed at a UK audience and is payed for by the license fee.
BBC World is aimed at a worldwide audience (with no bias) and is funded by advertising.
So it would be impossible to merge the two.
Also BBC WORLD is more of an information channel rather than a news channel. Sometimes the news is shortened to 5 minutes on the hour to make way for a documentary. Its not a rolling news channel like News 24. (Unless a big story breaks)