Tyreek Hill is a WR that can play RB. Cohen is a RB that can play WR. The important part is that they can stretch the field. It opens up things for other players.
Both can play at the same time. They don't have to operate in the same space. IMO you should see more two back sets during the year, with one or the other RB splitting out wide or Montgomery blocking inside sometimes. I think the Bears are going to try to be creative and unpredictable. Most of that is going to come early in the year. And then, in reality, everything comes down to matchups and game situations as far as who gets the ball.
So, one last time. I think Cohen scores more fantasy points than last year. I think Montgomery will score more fantasy points than Howard did last year. Overall, I think the Bears offense plays better statistically, in their division in particular. Both WRs are much healthier and are entering the second year in the offense. Robinson in particular has had a great off season. The TEs are coming in healthier. And, of course, the line is intact. The whole team is stacked right now.
If we are asking who will get the most fantasy points in the Bears backfield over the year (barring injury), I would guess Montgomery (but close). He should score TDs inside the redzone and catch some balls.
But I don't see that as meaning that Cohen won't be equally or more effective than last year. I expect to see Cohen utilized often early in the season. I expect Montgomery to be used more in the second part. It is a long season. And Cohen is currently named the starter.
So that is where I see the real value in Montgomery for fantasy - the second half of the season. He is built for bad weather. Personally, I don't see the Bears rushing him in. Too much to put on tape to spread the field. I think when the weather turns is when you will see the team lean on Montgomery. Like they tried to do with Howard last year. IMO That is when Montgomery will earn/be named the starter. But early in the season I expect the Bears to throw the ball all around the yard. To put that on tape before the weather turns. And that probably means more Cohen.