I don’t have any data to back it up, but maybe a good numbers of players acted as I did regarding GLs: in order to be able to get one each time a couple is released, I choose to unlock only one each time. It allows me to have access to a shiny new toy each time a new couple shows up.
If prerequisites for a GL duo would strongly overlap, it would be very tempting to relic 5-10 extra characters to unlock the other GL. So it could very well be a new strategic decision from CG to overlap prerequisites inside a GL duo on purpose, in order to motivate more players to go for both of them.