How to get two-seats mounts?

Hi there, I usually see some people in their mounts with a 'hop on' mouseover icon. I've even seen a D.R.I.V.E with this option, but I don't know how to enable this feature. Is this a mount specific feature? Which mounts are able to do this? How can I enable D.R.I.V.E to do this? Ty in advance!