Split a big file into two and then merge back into the original
I would suggest, to get performance and still keep the original intact, that .net libraries are probably not the best choice.  

One suggestion that would work, if you can allow smaller chunks(8GB), is 7-Zip.  It's free, open source and includes a file explorer.  It will split a 32GB file into 5 volumes in about 20 min.  Anyone with the same software installed can combine the volumes back again.  It even includes the option to calculate a checksum for the file to help confirm it's still the same file.