Bonkey
They definitely do split people up. I have flown with my friends EDI-STN on the same booking and I was in row 2 with my mate down in 22. Flight was about 75% full. The return flight had us together, but still one behind the other.
I've also had the opposite. On 2 separate bookings I've had all 3 of us magically assigned a whole row together just be checking in simultaneously on 2 devices.