I have reguarly extended PL stays both using PL and traditional booking channels. No matter what you do, the reservations will still be separate, so the most important thing to do is inform the front desk at checkin of both reservations so they can attempt to keep you in the same room for the duration of your stay and perhaps avoid a separate checkin on the last day.

I have never had a hotel force me to move rooms, even when the 2nd reservation was created after initial checkin. Just explain what's going on and they should be happy to accommodate.

Lastly, note that PL always offers to extend your stay at the same hotel for the same rate, but this does not reflect guaranteed availability. PL may still deny your request for an extension and offer a higher rate if the rate you booked is no longer available.