With my Philips I can edit the recording.
What I would need to do is 'hide' the last half of the recording.
I now have the original recording on the hard drive - say 3 hours, and the edited version at one and a half hours.
Burn the edit to disk 1. When you are sure it has burned ok, delete the EDITED version ONLY from the hard drive.
You still have the full recording. Now 'hide' the first half.
You will now have the original 3 hours and the edited version - which will be the second half at 1.5 hours.
Burn this version to disk 2.
I hope I've explained this clearly - it does seem complicated if you've never done it.