LOTS of candidates come back to questions. The fact that the second half of your answer is in a different place, maybe a different booklet, from the first half will make no difference at all. Even if the marker for that question misses it, the checkers won't miss it - the checkers go through each script to make sure that everything has been marked (and that the marks for each question are correctly added and transcribed onto the front cover, and correctly entered in a spreadsheet). It may in fact be picked up before it gets to the marker: we have people who open the envelopes and check that the `questions attempted' listed on the front cover correspond to what has actually been attempted. The markers have a special notation to indicate that the answer continues at a later stage.

I'd prefer it if candidates wrote on sheets of paper, rather than in booklets, and stapled the sheets together in order at the end of the exam, with the invigilator checking that it is done correctly. But the exam board thinks that the risk of mistakes would be greater, partly because it is different from all other exams. Maybe they are right.