among x between; each other x themselves

[beachboy]

There were 4 senators in the meeting. They couldn't agree
- among themselves
- among each other
- with each other

There were 2 senators. They couldn't agree
- between themselves
- between each other
- with each other

Which sentences are right?
Can "each other" always be replaced by "one another", even in other cases?
Thanks

[Anglika]

First, which do you think should be used?

"with each other" could be replaced with "with one another".

[beachboy]

I just wouldn't use "themselves" in either case. I think "each other" and "one another" mean the same in both cases, but I can't think of a situation in which they aren't interchangeable.

[Anglika]

I would use "among themselves" for #1 as there are more than two people involved.

#2 "with each other" as there are only two people involved.

"each other" implies only two individuals, so "among" is incorrect.
"between themselves" is a collocation but I would not use it in this sentence.