Note: I only speak about Windows.

If you have a text file, then each line should end with \r\n or \n.
When you open a text file in text-mode, each \n will be replaced with \r\n. When you open the file in binary-mode, no such conversion takes place.
Conclusion: When you do not want conversions to happen behind your back, open the file in binary. Because you said you wanted to split the file in 2 and then merge them back in one, you want the resulting file to be exactly like the original, don't you. Therefore open the file in binary, and no conversion will take place.