My primary read mine first then secondary. As the secondary virtually only did grammar checks after the primary had asked for extensive revisions, I dispensed with his services towards the end, once I'd learnt and incorporated his grammar style as part of my own writing style. The primary was that thorough I was able to get away with this.
The secondary was just happy to be left alone.
Go with whoever gives you the highest quality feedback, though out of politness keep the other(s) in the loop and hand drafts to them if they have a decent contribution to make.