Only other thought would be to change the button div from
<div class="submitbutton">
to
<div class="submitbutton clear">
This will place Split button at the bottom of the page
or
<div class="submitbutton clear_right">
This will place the button at the bottom of the right side "selected" column, which may be the best place for it really
Also the script is thorwing an error topic.js when you select the arrows to add to the split list.
TypeError: XMLDoc.getElementsByTagName is not a function
pageIndex = XMLDoc.getElementsByTagName("pageIndex")[i];
http://localhost/elkarte/themes/default/scripts/topic.js Line 1145
Can't look at that one ATM