Exactly as Palal said. IME I've had Pax on the same PNR seated next to each other as auto-assigned on check-in between T-36h and T-24h. (so they don't actively separate Pax. for now :-) )
To be sure ahead of this time you'd have to make a seat reservation. During the auto-assign if you are not happy with your seats you can always pay for what is still available (note between T-36h and T-3h if you want to move seats you still have to pay for them)