It is possible to re-cast a Green Magic spell (e.g. Harden on an Acorn) while the first spell is in process. Each cast counts as an individual use of the skill for skill progress purposes.

Further, having cast Harden multiple times on the same acorn, when picking up the acorn I did receive the message about it being destroyed, but the acorn remained on the table with the enchantment animation playing. (I did not receive the acorn into my inventory.) The server hiccupped before the spell(s) resolved, and when I logged back in the acorn was gone, so I'm unsure whether it would have also vanished as intended when the spells resolved or not.