Ropes have to hang on something (it's one of the well-known things that ropes do), so when you say this:
A rope was hung between two poles.
you strongly imply that they are on the poles, either tied to the poles directly or put on a hook on the poles.
Could it also mean that the rope was situated between the two poles, but was hung on something else not mentioned?
Why else would you mention the poles in this sentence with the rope? If there is a good answer to that due to context, then yes, but otherwise no.
A rope was hung on two poles
There's a chance someone could assume you really meant "A rope each was hung on two poles" - meaning two ropes, two poles, other end of rope on the ground. With between this wouldn't happen.