They'd have to record about 6 hours worth to get a half hour program that would actually be watchable.
I would rather give them 45 seconds to provide a soundbite, then another couple of minutes to follow up their point, then another 45 secs to conclude. I would cut their mikes at the end of their time.
I'd also keep the camera on both, because it's good to see their body language and responses to each others points.
Watching Sunaks lack of control in shouting over Starmer, compared to the Starmers technique of putting his finger and thumb together to create pressure as a way of focusing his frustration was illuminating, you will miss things like that with a pre recorded program.
They could have fact checkers there and after each break, recap the previous segment before the debate starts again, it might extend the program, but that wouldn't be a bad thing.