Other mistake: In the scene where Duncan has been captured by the Watchers and restrained in the upright table, take note of the needle. The camera pans around the Watcher's head and then shows the syringe delivering its contents. Once the dose is fully administered, it can clearly be seen that there is no needle on the end when he holds it down near the Highlander's chest.
Other mistake: As Kate/Faith is walking to meet Duncan, there are cars parked at the side of the road,the scene is meant to be in London but the number plates are the wrong style for the UK.
Answer: The scenes you mention were only meant to be part of the trailer. This was to show that Jacob Kell was the "most powerful" immortal yet. The 'cut in half' scene is playing on the idea that Kell is unbeatable. I suggest rewatching the trailer with these ideas in mind. It may make more sense. These scenes were what Dimension Films wasted a hefty portion of the promotional and marketing budgets on. There was NEVER any script written that included them, nor any version of the film that ever included them.