To be honest, it's a bit of a faff - If your machine can handle running three versions of elite at the same time (mine... sort of can but.. I mean, the frame rate drops... a LOT) then it's not too bad, because you can just switch windows and stuff to remove/reform crew (can't use outfitting, change ships, etc with a crew, which is DUMB by the way!)
But if you are running on multiple machines?
As I say, bit of a faff to keep doing that.
I had a go a few months ago making Operation Ida runs with CMDR Raiko flying and CMDR 'Fat Tony' Lonnigan as co-pilot for fun, running on the same machine. I put the main account on "Medium" settings and Fat Tony on "Low" which fixed most of the frame rate problems.
It was fun actually having two seats filled in my Type-9, but still had issues. I think I caused a bit of elastic banding at the repair station playing in Open - might not have been me as it was very flaky anyway at that time, the second account also often disconnected during hyperspace jumps.
As you say, it is a lot of faffing, I'm not sure that I'd use if for anything more complex than cargo runs, mining and perhaps missions.
I really hope that FD eventually allow NPC crewmembers to appear in the multicrew seats. Maybe with Odyssey at some stage, as they're planning to finally have NPC interaction (I think).
... or .... 
.... or even 
