Hey rupert413. As I see it, you have two options.
1. Format the drive in the exFAT file system. This file system is fully supported by both Mac OS X and Windows. This in my opinion is the better option, because you don't need two separate partitions and don't have to be restricted with 1TB for each, in case you need a bit more space for the files from one of the OS'.
2. You can create two separate partitions for the 2 operating systems. One NTFS (for Windows) and the other HFS+ (for Mac OS x).
Here's a guide on how to format the drive:
How to format a hard drive
Just click on the corresponding OS for detailed instructions.
Hope this helps.
Boogieman_WD