The TX802 can have up to 8 different voices loaded at the same time. But due to MIDI limitations on the synth, it's only possible to select which voice is being edited on the synth itself. To select which voice you are fetching or editing:
Once you've done this, when you click the "Fetch" button in Patch Base, the current voice from the selected part will be fetched, and any edits you make will be sent to that part.