Great question and answered/addressed a few times. Aside from personal preference, there are reasons other than preference that some use two hand.

The reason usually involes correcting a difficult razor or problemed edge. In this case, I've copied a technique I've seen gssixgun do and I'm sure many other honers have done and it has been called "pushing the edge". The why's and how's I'm not going to get into, but there are videos and descriptions on the topic.

-Just my little addage- YMMV and Have Fun!