I just did an update which the forum threw away ...
Looks as if you need an @-epubx-partition rule to define properly flowing two column,
probably (but ???) to replace div.box-1
http://www.idpf.org/epub/pgt/#s3
General opinion on the web seems that it is best to avoid 2 column as most ereaders have pretty small screens.
May not apply if you know you have just one target mahine with a largish screen.