Southwest Airlines offers a service called Early Bird Check-In. For a fee ($15–35/person depending on flight) you can ensure that you will be among the first to board and therefore first to choose your seat. (It's an open seating policy.)
While booking an itinerary on southwest.com, there is no way to only purchase Early Bird for certain passengers or certain legs. Selecting Early Bird will purchase it for each passenger and each leg, which can quickly add up.
Lately, when booking flights for 4 people on Southwest, I have split my booking in two, selecting Early Bird for two of them and not purchasing it for the other two. Obviously, I get two different boarding positions when checking in, but the two people with Early Bird save a seat for those who don't have it.
Whenever I do this, it feels unsettling, probably from all the stories on this site about itineraries booked separately. Is there any way this could reasonably backfire?