So today I was farming argus rares for the mounts, and I parked my main in front of the rare after I killed it to log into my alt to kill it there as well. Which I did, but after I logged back into my main to continue my day but I saw that I could kill the rare again, and I did, and then I logged back into my alt, and I could kill it there as well.
but after I did it a couple of times, it seemed that it didn't work anymore but I did get both mounts.
The rares I did it with was "Wrangler Kravos" which drops "Maddened Chaosrunner" & "Skreeg the devourer" which drops "Acid Belcher"
So to sum it up.
You need 2 characters.
With your main character go to rare, kill it and logout
With your alt go to rare kill it and logout
Log back into main, and kill it
And rinse and repeat.
Let me know if it worked for you!