That was one of the issues that David Lynch's Dune had...too much to say (going by the Novel) in too little an amont of time, and many things ended on the cutting room floor!!Yes, due to the story he is telling, the first film, or first half of the entire story, has to play out in that first couple of hours. This is a case where I would find it cool if Denis cut the film into one long feature for home viewing. I don't mean it as a complaint because I think he is behind the 8 ball making the first film. Sure, the maximum length they really want a feature film to be is no longer than 2 hours and 17 minutes. So, if you have five hours of story, well, yes, that has to be more than one film. Dune is a dense and complex universe with a lot of things to lay out for the audience and so that first film, part one, is hard.
He does have the advantage that he is working from a novel, so the story is all mapped out and he knows what the ebb and flow is and needs to be. I get the sense that watching Dune 1 and 2 back to back, if you have the 6 hours to spare, will be the most satisfying way to view it. I mean the second half of the story is going to ramp way up. This is where taking novel to screen can be complex and why Dune is not an easy story to do that with.