Assuming you are not both assigned middle seats (which tend to be less preferred), in most cases once on board it is likely you can swap a seat with someone with an “equal” seat (aisle, middle, or window) and sit together. If it is absolutely critical that you sit together, you should pay the upfront fee.
In mid flight I moved from my seat to an exit row seat (the ENTIRE exit row was empty) and was told by a flight attendant to return to my seat. I thought this was pretty chintzy, airline had nothing to lose by letting me sit there. They’d rather see the seat empty than make a better flight experience.
Regarding trading seats after boarding, I don’t buy (pun intended) the argument about balancing the aircraft, they don’t know what you weigh when you buy your ticket and assign the seats in the first place.
Used to be if you wanted more space if you had two people and ordered the window and aisle seat of a three seat row there was a good chance you’d have an empty seat between you. Airlines now are pretty good at making sure all flights are full, so that doesn’t work as well anymore, except perhaps on red eye flights which are less likely to be full.
Then there was the story of the vulture that tried to bring two dead armadillos on the plane with him and was denied. Only one carrion item per passenger.
