They can if they have enough monitors.
Oh, I think the "disused" platform is actually platform 6. According to realtimetrains, 1 train used platform 6 today (the 9:04 to Cambridge North), and 5 used platform 5 (all northbound). This compares with 92 trains for platform 7. While a single person can't see both sides of a train at once, whether or not the doors are open, it helps a lot if the island on one side of the train is deserted.
Really, though, the best thing to do to allow both sides of the train to be used would be to make platform 6 exit-only for some trains, and block off the stairs until the train arrives to encourage compliance. Then all you need to do is work out how much time using both sides would actually save