NIN + BLM vs MNK + WHM is a fair comparison in many situations. BLM is rarely adequate support for a MNK, but it is often adequate support for a NIN. So if you're intent on duoing, especially for AF3+2 items, then NIN is much better for this not only because of Ruby procs but also more Amber procs due to the conversion of a WHM to a BLM.

My team of friends is usually NIN WHM BLM. This is very efficient for nearly every activity in the game. If there is a situation where we want Azure procs, I simply switch to MNK after farming the pop set probably on NIN.