Is it adviseable to stack internal hard disk over each other (after removing them from the casing)

Solution 1:

For storage, the issue would be them sliding against each other and the top of one drive damaging the circuitry on the bottom of the drive below it. A cardboard spacer would solve that problem.

Static electricity is a risk in theory, but I've abused hard drives like this for more than a decade -- I have dozens stacked -- and have only had one that I put away good not work when I unpacked it, and that was a 20MB(!) drive that had been stored open in a moist garage for a decade. Still, putting them in an anti-static bag is a good idea if you've got one.

For operation, the issue would be the drives getting knocked over, shorting each other out, or overheating. For short term operation if you're careful, there's no real problem. For long-term operation, there's the possible issue that vibration might cause gyroscopic effects that lead to torque on the bearings, causing them to wear prematurely.

Solution 2:

As long as you don't bang them up, and no metal is touching the circuitry that shouldn't, you should be fine

Worst case scenario? Put them in a giant condom