Split External HDD that already partitioned

I want to ask, I have a 1T External HDD: 250GB for data, 250GB for photos / videos and 500 GB for Time Machine Back Up. It was already set for it from beginning.
How do I split the External HDD that is already partitioned 500GB to split into 2×250 GB to be used for other purposes and without deleting Time Machine backups data that already exists on the 500GB ?
