I run around with two toons using a single mount probably more often than I play a single toon, so I have quite a bit of experience. Yes it's a bit buggy between zones, like BJ already mentioned though, most of the time it appears to the "driver" that the passenger is gone, but from the passenger's perspective they are still there...so when you dismount it'll look like the passenger just ported to where you are.
I've heard the horror stories of deaths but I've NEVER had a death related to this...and I've done probably hundreds of mounted hours with an alt. I'm sure it does happen, but it's gotta be incredibly rare...unless it's lag-related or something.
I created an add-on/macro combo to have a targeted player automatically accept the mount request, which helps alot...though it does inexplicably fail every so often. Overall though I'm very happy with the 2 person mount idea...extremely helpful.