An offset account can only be linked to one loan account. So one offset per loan. Link the offset to your PPOR.
If it does not cost you any extra dollars, then there is no harm in having an offset for your IP as well.
Here's how they work:
Your pay goes into the offset account.
The offset account looks, feels, acts in the exact same way a normal day to day transaction account does (well at least Westpac's one does).
Whatever amount is in the offset account reduces the interest charged on your loan.
For example, if you have a $500k loan, but have $100k in your offset, the bank will only calculate the interest on $400k.
So the more money in the offset, the less interest you pay.
For this reason, it is best set against the PPOR. Your IP is tax deductible, so you would be a fool to reduce the interest charged on your IP. (That said, if all you have are IP's then offset the one with the highest interest).
Now spend everything on your credit card where possible (but not to the point you spend more than you earn cos that would be stupid). At the end of each month or cycle, clear off the credit card with the money in the offset. This way you maximise the amount in your offset to reduce your interest charged.
Offset Accounts are NOT the same as redraw facility.
From a cashflow perspective and how it affects the interest being charged on the loan, they are identical (well to you the customer anyway).
From the ATO's perspective they are very different.
Using the example earlier with the $100k sitting in the offset and a $500k loan, the bank charges you interest on $400k only.
If you paid the $100k directly into the $500k loan, you would also be charged interest on $400k only.
Heres where the difference is:
Offset Account
If at any point, you decide to convert your PPOR to an IP, having an offset will allow you to withdraw that $100k (and use it for any purpose - eg buying a car, going on holiday - ie personal use) it does not affect the tax deductibility of your loan - ie the $500k is tax deductible.
REDRAW FACILITY
However had you paid the money directly into the loan and redraw it back out for personal use, only $400k would be tax deductible. The reason being is that you actually PAID DOWN $100k on your loan.
For this reason I personally believe it is 100% better to use an offset account over a redraw facility any day. (The exception to this rule is if the redraw facility offers a substantially lower rate, but these days i find the rates pretty much the same rate).
Now if you have ZERO intentions to convert your PPOR to an IP, then redraw or offset account make no difference
NOTE: My assumptions are that the offset account is a 100% offset account. I bank with Westpac and mine is a 100% offset.