I dont think it's going to be ???-> Thief and Nin. cause if you look at SMN/SCH, they are different roles. So i think other dual jobs will be different roles too.
This is where those Hymn items come in too; the jobs use different stats, so you cant be optimal on 2 jobs. So you kinda need to pick a stat, and be a little gimp on the other.
So i think it would be more like a str job, and a dex job.
Also i think it's more likely to be that *brotherhood* that will be class guild.