Time machine backups and file storage

I have an iMac and a macbook pro. Looking to get the 5T LaCie external HD. I want to use it to store files AND back-ups for both computers. I'm planning to keep it connected to the iMac and share over my network so that I can access the files and remotely backup my macbook. I know that I don't have to partition the HD for the separate backups because the software developers of Time Machine are genius ;-) However, I'm thinking that I do need to partition to have a separate place for the files (mostly pictures). Is this correct?? Or is Time Machine doing this automatically and I can skip that step?