Having researched the 9th NY very heavily the monument placement is simple in the details.
The spot where the 9th's marker is where they suffered the most of their casualties on the 17th. It is where the Color Company (E) was pretty well smashed by Drayton's Brigade. From there they crossed over to the swale behind and some of the men attempted to get to into the town. Several were found in an orchard on the outskirts of Sharpsburg.
The 9th eventually rallied and moved back to the knoll above the Branch Ave pull off overlooking the Otto and Sherrick farms. From there they waited for reenforcements to come up to them, but were forced to retire late in the day.
The 8th CT was sent to aid Fairchild's Brigade, which the 9th was a part of. One company outflanked the Pee Dee artillery and drove off the gunners to the south of Fairchild's Brigade, but soon abandoned the guns and rejoined the regiment as it pressed forward. They came to where Fairchild's Brigade had crossed the stone and rail fence and redressed their lines. The 8th CT was alone as Fairchild's Brigade had moved forward to the village and had returned by then, but there was no one to guide the 8th CT so there they stayed. Unfortunately no reenforcements came up to them either and they too were soon withdrawn in the face of Hill's Confederates.
So that ridge is important for both units. Hence the monuments placements.