Well, why not? I don't see any particular problems with the combination flavor wise. There are weirder things you can make work that are still perfectly reasonable. I don't see the argument "they're too similar" as a good reason to disallow multiclassing.

That said, I'm going to ignore the OP for a moment and say, yeah, the crunch for it sucks. What's the point of doing it in the first place, anyway? As far as gish-types go, the cleric class is practically a gish already if you want it to be.

The setup isn't going to break the game, unless the other players have significantly more optimal builds, in which case the class is going to be a bit of a deadweight.

Grey Guard, however, I have plenty of problems with in terms of flavor, but that's not what the question is about.