Well, I guess since this was obviously a mistake I don't really need to put in my two cents but will anyway. I play on PS3 and was unaware of the L1 R1 meathod, but I have always placed all my mounts on hotbar 5 which is tied to R1+Dpad Up so for me its as simple as a quick switch to get to the bar and just hit any mount to dismount the one you are currently on. It's not too big of a deal, just takes some getting used to.