Best to ask a tax agent or ATO's technical assistance or private ruling
Generally, claiming interest expense is based on ownership of property and generally, having extra names on the loan is not a problem.
Reading Example: joint borrowers, sole owner – claiming all interest incurred may help
Though the part that I'm unsure of is that both of you are contributing to the loan. This might be treated as a loan from the non-owner to the owner.
- ATO ruling TR 93/32 - paragraph 49 - "The fact that Mr Z has paid all the expenses on the property is of no consequence for income tax purposes. We would simply treat the payment of Mrs Z's share of the expenses by Mr Z as no more than a loan by Mr Z to Mrs Z."
- https://www.ato.gov.au/law/view/document?DocID=TXR/TR9332/NAT/ATO/00001