The only way multiboxing gives you more chances at a mount is if you are in the open world doing Sha of anger or bosses like it. Within instances multiboxing does not improve the chances in any way.
I know this because as a boxer I made a ticket to ask.
Irrelevant, it is not players that decide if something is “legal” or not. They can only say if they like it or not in one of the dozens of threads on the subject.