HAVE ONLY TESTED WITH DK AND DK FLYING MOUNT!
You must have DK flying mount to test this
1.
First get to lvl 77+ and go to Storm Peaks (K3) where you buy certificate to fly in Northrend. Buy the certificate.
2.
Get the "Loaned wind rider reins" from "Honest" Max right next to where you buy mount certificate.
3.
Get on the "Loaned wind rider reins" and then mount Dk flying mount, now you are on 2flying mounts =)
4.
If you fly up a bit and then right click on the "loaned wind rider reins" in you inventory then both mounts will disappear and you will fly like a bird.
2 mounts
none
ps: stolen content