They could look at 70% LVR low doc at good rates and split into 2 loans one for current amount and one for the amount you will effectively be lending you. Tobe may be able to assist with this.
I think Westpac and St George limit the guarantees to 1 per borrower?
I think the OP is talking about getting a cash out loan against the parents property rather than just using the property as additional security? This may be a bit tricky.
