I was faced with a similar situation, to my knowledge you can only do this if you don't actually purchase a second property. So if you rent out someone elses property, you can claim your own property as your ppr for up to six years (even if you're renting it out at the time). As soon as you buy though, you have a six month grace period (CGT exempt) to sell the old property.
If you move back and forth between living in properties you own, I don't know what exactly happens. At best, I think you would split the CGT based on how long you lived in one property (so 10%/90% split, if you lived in one 10% of the time). More likely just the one you live in the most is considered your PPR.
