|Static bags have no affect on magnetic fields. It would, however, be a good idea to put them in static bags to protect them from, well you know--static.|
Hard drives do have magnets in them so I suppose it's possible the magnets in one drive could affect the contents of another drive.
In computers with more than one hard drive, they're often mounted one on top of the other with very little clearance between them so I don't think it's a big problem. I have dozens of drives I'm not using stacked on top of each other and haven't noticed any problems should I pull one out to use in a PC.
But some separation couldn't hurt. You could probably store them separated by an inch or two and be safe.
Oh, another thing, the placement and shape of the magnets will give a non-uniform magnetic field. I'm guessing, but it may be safer stacking them on top of one another--with some separation--than if they were close together in a different configuration.
You're not really green until you're soylent green.