Combine base resource with international resources

I would like to combine the base Patient resource with the PatientAU resource. How do people combine elements from 2 different patient resources into one?

So I am asking if I should have a single patient resource and combine elements from AUPatient or I should have 2 separate Patient resources one for AU and one for base.

You’d have a single instance. All instances will comply with Patient. Some will comply with AUPatient. Some may well comply with AUPatient, US Core Patient, NL Patient and various others. So long as you include the mandatories required by all profiles and include the codes required by all profiles, then it’s straight-forward to have a single instance that can be used in a wide variety of environments.