If you really want to break the text into pieces, using LEFT and MID (as repete suggests) will work, but gets a bit clumsy to manage if you want to split it in more than 2 pieces - that is, for a very long string, split into 20 per cell until it's all used up???
Is splitting it into separate cells really what you want, or would turning on wrapping so that it displays on multiple lines in the same cell serve the purpose?