I am unaware of any "linked" creatures that have been published on the PFSRD. There may well be one or more, but I haven't come across them.
That said, this is an awesome idea, and definitely one that shouldn't be hard to build:
- find your favorite creature
- add the zombie (or, perhaps, the bloody skeleton) template of your choice
- separately, add the ghost template to the base creature
- for effect, set the encounter on unhallowed ground (it'll also help alert the players that something weird is going on)
- determine how, how often, and how quickly each half of the creature can bring the other half back to undeath
- add an appropriate CR adjustment for the Unhallow and "Resurrection" effects (Unhallow is probably worth +1 by itself, as unfavorable terrain)
Admittedly, that last step is hard. I can see arguments from 0 adjustment (it takes at least an hour for one portion to bring the other back) up to 3 if it's a quick restoration of the other half, and that other half comes back at full HP/abilities/....
Off the top of my head...
CR +0: the surviving half can bring the other half back, but the process requires an hour of concentrated effort. This is the "flavor" option: it's somewhat unlikely to come up unless the ghost part goes and hides (thus allowing the PCs to overcome the challenge) or the zombie half is a bloody skeleton and the PCs don't manage to render it incapable of coming back in an hour on its own.
CR +1: the surviving half can bring the other half back 1d6 (or 1d4+1) rounds later as a standard action; the restored half comes back with half of the restoring half's HP (round up).
CR +2: the surviving half can bring the other half back 1d4 rounds later; the restored creature has half normal HP.
CR +3: the surviving half can bring the other half back 1d2 rounds later; the restored half has normal HP. This is devastating if the party doesn't have an extremely good way of dealing with the ghost's incoporealness.
Oh, and one more thing: determine why the halves stick together. If the halves can remain "alive" and well forever if they just stay apart, why are they sticking together? ... especially if one of them is mindless. Even if the PCs never figure out why they stick together, that knowledge should inform the undead pairs' strategy and tactics.