if changing only 2 new tires at a time, should I place the new tires in the front or rear on the Cayenne?

 

why are you changing 2 at a time?
buy a full set. its much safer than driving with warn out ones in the front and new ones in back or vice versa
i would only change 2 at a time if the front 2 or back 2 are excessively worn out

 

because I replaced 2 tires 3 months ago (they were excessively worn out), today I replaced another set.

 

Normally, I would put new pair up front because newer tires should have lesser chance of failure. A front tire popping is much more dangerous than a rear tire due to loss of steering. But in your case, I don't think it will make any difference given your other pair is so new... That is unless you have driven tens and thousands of miles in the last 3 months.