Since year dot 3PAR systems have only allowed one drive to be replaced at a time (unless replacing all four is a single Mag).
Servicemag resume needs to have completed on the 1st disk before you can start a servicemag on a 2nd disk. That's the only way I've seen it done and that includes when we used to occasionally get one of the top 3PAR engineers in (who went on to be head of remote support after HPE bought them
).
Bad things happen if someone forgets to do logging or pulls the wrong mag from my experience.
There are a few bodges that can be done but I'd not go there unless no other choice (i.e. the array is already offline).