Hi @info289,
Allow me to share how QuickBooks Online (QBO) works, with regards to ABN.
QBO by design calculates GST for a single company per subscription. The feature to divide this tax is currently unavailable.
You can either subscribe to another QBO subscription to record the GST of a different ABN, or get QuickBooks Desktop. This allows you to create many companies using a single license.
For a quick introduction of available GST settings and what they do, you can view this article: GST Settings Overview.
Drop a comment below if there's anything else I can help you with. We're always here happy to help.