Interest on any funds released from your investment property cannot be deducted as an expense against your rental income, so this is not a tax efficient option.
i actually hadnt suggested releasing funds on the investment property to benefit from a tax perspective i suggested it as the current rental imcome greatly exceeds the interest on the mortgage so the OP can afford to increase the mortgage on the IP and still have the rental income cover the mortgage, the flip side to this would be the mortgage on the PPR would be further reduced thus the mortgage payments/term on the PPR will be further reduced, given cash flow will be an issue for the OP i'd focus on reducing his expenses/outflows to more manageable levels.
i'm not saying this it the right or only way to do it but its the way i'd choose based on the OP's thoughts (if it were me i'd sell the IP immediately and reduce the PPR's mortgage as much as possible)