I do this all the time. I always book paid flights together on one PNR, and then can manage the flights together and select the preferred seats for my companions. Then I'll call SMS and have them split out my ticket to a new PNR so I can be eligible for the upgrade (usually sometime before the 5-day window although yesterday I called for a flight on Saturday, and the upgrades cleared as soon as my record was split out).

If for whatever reason I don't have the tickets on the same PNR (e.g. booked with vouchers. or different flights in one direction), I'll just call SMS and ask them to seat my companion(s) next to me in premium seats.

Of course, my son is not 15 yet, so we can't book the exit rows unless there are at least two adults flying. For example, for upcoming itineraries, on some legs I booked my son and his au pair into premium non-exit seats, and my wife and I into nearby exit row seats.