I thought I read somewhere (although I have not been able to recall the source at the moment) that SG1 and SG2 received very similar retainer salaries. What makes SG1 referees earn much more was that the fees per game were almost twice as much in the Premier League compared to the Championship.
If that is the case - anyone know for certain? - then there would not be any financial reason for not being able to combine the lists (as you would just have more flexibility in allocating the same match fees). Of course there could still may be a contractual or HR obstacles.
Those at the very top would still earn much more. Someone like edy would know this, but I think our FIFA elite refs earn something in the region of €5,000 per match in the UEFA competitions.
When the SG2 was officially announced the basic salaries were 50% of the SG1. Have no idea if that is still the case. Match fees vary as well per league and role.
FIFA matches vary by competition and round, it's not a set fee per match but rather a sliding scale.