What about exporting the content from culturalinfusion.org.au/soundinfusion and importing it into a newly created category "soundinfusion" on your culturalinfusion.org.au/portalnew site? Then create a different header for this category, so it reflects the "sound in fusion" focus. You can call different widgets, with custom menus, specific to this category, too. This would all be under the same theme of course, so yeah, would probably make your life easier.
Multisite is awesome, but may be overkill for what you need.