There are hundreds of File splitter and merger utility available for you to choose and use.
But only a few support Unicode. If you try to split Unicode text files through non unicode supported tool, your data may lost and all you end up collecting Garbage.
Here is a nice utility called JR Split Files that you can use to split / merge Unicode text files. The tool is tiny one, merely of 170 KB. Simply download from here, unzip it and start using. No installation required, and the interface is pretty straight forward.
You can ALSO define in how many parts (byte size) you want your files.