Just a minor addition:
While hard disk magnets are strong enough to make me salvage them from broken hard disks, they're also mounted so as to "counteract" each other. If you take one of those magnet pairs as it is mounted, and try its effect on items outside the enclosure, you shouldn't see much effect. Only when you tear them apart from each other and use the single twins of the pair will they show their true strength.
The magnetic field generated by the motor would seem more troublesome to me, but that applies to CD Drives and the like as well.
Normally, shielding and the, albeit little, distance between the drives is enough to keep them from irritating each other. As already mentioned, heat or the risk of falling is much more of an issue, as is dust when it clogs up the vents of the enclosure.