There is Getting Started course in pipe line that would explain you how resource selection is done. But for now I suggest you raise a case with NGS and enable some extra logging to find why the other filer is being ignored.
At a high level for Volume SnapMirror following are done.
The ONTAP version of the source filer and the root volume language of the source filer (language of the mirror source, if there is no language for the root volume).
The order in which these aggregates are considered is as follows (lower the number, higher the precedence):
- Same ONTAP version, same language
- Different ONTAP version, same language
- Same ONTAP version, different language
See if the above helps.