The entire basis of the plan was to trap the confederates in a pincer, with no easy escape routes, and force them to fight. If splitting his forces to come up two roads was too risky, then Rosecrans' plan was a bad plan from the start.
I don't think Grant ever completely bought that "supporting distance" excuse. As Grant pointed out in his report, while it was true that the two roads began spread out from each other, they rapidly converged as they got closer to Iuka.
As compared to what? Doing nothing? The Army of the West had moved to Iuka in order to harass Rosecrans forces and prevent him from sending additional forces across the Tennessee river via Eastport and/or a Pittsburgh landing. They were never going to be able to be in a position to contest substantial elements of
both the Army of the Mississippi and the Army of the Tennessee, nor did they have intention of doing so. Once sufficient information was provided from the September 16th operation against Iuka via Mower's brigade (what had been Murphy's brigade), it was clear that there was a real opportunity
if U.S. forces could move fast enough and get a little bit of luck. It was always something of a long shot, but it was
possible and if you never take shots, you will never hit one. There was a real opportunity here, again, even if something of a long shot.
When you understand the situation properly and in light of the size of Price's force, I think that it becomes pretty clear how difficult this was going to be and based on the situation as he confronted it, Rosecrans
probably made the right decision in moving via the Jacinto road alone, but it was a difficult decision at that point. They were going to have to pitch into the Army of the West hard regardless and we have covered some of the basic nature of the situation as it actually existed here already. Had they moved via the two roads separately, they would have probably be hit in force, most likely with
all of Little's Division, or possibly with both Little's and Maury's Divisions combined on one of the two roads. Had they pushed to get to the Army of the West that day/evening, they would have fallen back via Bear Creek that night. Had they been able to get to the Bay Springs Road and move laterally upon it and then pitch into them, especially if following the attack from Ord's men and could have turned the left of the Army of the West between them and Bear Creek, you never know. It is
possible that they could have caught them. They then would have most likely still had to destroy them, but it was
possible. Again, it was a long shot that way.