OK, I'm not an electrical engineer but from my limited electrical engineering 101 training many decades ago here's what I think I know.

Running fuses or breakers in parallel for load sharing will increase the point at which the circuit protection (fuse or CB) trips if they are of equal amp rating. Lets say we need a 120A fuse but can't find one in that rating so we run two 60A fuses in parallel to achieve the 120A load rating. But load sharing is dependent on resistance so the load sharing will only be equal if the resistance in each leg is also equal. AND the wire as well as all other elements of the circuit had better be rated at 120A in this example or you are not really protecting the circuit.

TA, what you're suggesting is already below the 120A load from the inrush current so I expect the fuse to blow very quickly and then the 50A CB will be carrying the entire 120A load just like it does now. And the CB should also trip just like it does now. Please correct me but it seems to me a better approach is to mitigate the high inrush current that results from instant start of the fan. A soft start ramps up the current over time lessening the inrush. Another suggestion was to run the fan at the slower speed which will also mitigate the high current condition.

Or perhaps the current CB is faulty. CBs are usually slower to trip than a faster blowing fuse and can accept a margin of overload for a short time so can handle some inrush until the motor gets up to speed.