I propose the theory that a ghost isn't limited to rezzing one person, but rather can only rez one person [i] at a time[/i]. Jaren ward was unable to be rezzed bc of thorn and so was then able to link with shin malphur. I think if jaren ward was still alive, his ghost wouldn't have been able to do this. This theory fits with our ghost and the master chief situation (if you take that as Canon). It's also why our ghost couldn't revive cayde, bc we would've had to die for this to happen. I don't think there's any lore to support this, it's just spinfoil, but I think it makes sense with the exceptions we've seen in the lore
There is lore to support this.
In the Pigeon and the Phoenix lore book, Osiris tells a Warlord’s Ghost to go and find another Guardian.
Because Dredgen Yor’s Ghost abandoned him, Yor was able to be killed permanently.
As such, my theory is that a Ghost can choose a new Guardian if they either abandon their original one, or their original one dies.
Again, the lore on this is a little foggy but that’s the best sense I can make of it