That’s tricky one. Some cars were designed to have a single wiper for cleaning a good portion of the windscreen, if not its entirety. But yours was clearly built to have two and taking one of them off could be a reason to fail its MOT if the remaining wiper does not clear an area of the windscreen large enough for you to have an adequate view of the road. But this is very subjective and can easily lead to an MOT fail.
Since taking the wiper off or keeping it on can result in a failed MOT, your best option at this point would be to just have it repaired or replaced. I had a similar problem with a broken wiper on my VW Polo. I had mechanics at a garage here called Claremont Motors replace it for me, and they were kind enough to let me know that there were other issues like worn out tyres and brake pads that could lead to a failed MOT. The lesson is to get your wiper situation sorted out at a garage to avoid MOT complications.