Just make several widgets. Each widget can have its own UiBinder file. Then you can combine them at a higher level like:
#UiBinder file
...
<myWidget1></myWidget1>
<myWidget2></myWidget2>
<myWidget3></myWidget3>
...
Can you explain why you need the file broken out? Perhaps a messages (i18n) file/interface will work if you want to pull in dynamic content or settings.
If you're looking to pull in settings/strings/content from another file you can do this like:
#UiBinder file
...
<ui:with field='msg' type='com.foo.MyMessages'/>
<someWidget>
<ui:text from="{msg.someString}"/>
</someWidget>
...
Sincerely,
Joseph