Until recently, my husband and I followed this scheme: I buy a return ticket from the US to the UK from a US agent and my husband buys one from the UK to the US from a British agent. We are always one ticket adrift from each other, but when checking in, we check in together, as a married couple, and have always been seated together despite that my tickets are in my maiden name as I've not changed my passport.
This requires advance planning of course, so we know when to set the return dates. Also, we always checked the price of a one way, just to be sure we were getting the cheaper deal.
After doing this for 2 years, we ditched one of the return legs, just didn't use it, because it didn't suit our travel plans. Now we buy both our tickets at the same time, for the same outward and return journeys. In our case, we consider that we only lost about $200 dollars, half the full ticket price.